I haven't planted one yet (thinking about planting one this year in the backyard) but it sounds like an oakleaf hydrangea might be what you are looking for. They can tolerate shade and can get pretty big. Plus year round appeal.

Another discussion can be found at: http://forums2.gardenweb.com/forums/load/shrubs/msg062122079927.html

Cammonro is probably right, looks like oakleaf hydrangea the only one that might be meeting your criterias about height assuming they are established with no yearly dieback. All paniculatas could be even taller, but they would need much more sun in order to bloom. (BTW, PG could be found in both forms, form of tree or in form of shrub, depending how it was trained when young.)
There are some of H.serrata cultivars that will be in 6'range, namely Greyswood and Woodlander, but they are lacecaps and are not as showy as traditional mopheads, though for a natural setting they look more appropriate than mopheads.
Here is the 5' tall hedge of another serrata, Blue Billow

Hi Ego45 - Happy Spring! Would your beautiful Blue Billow and the Greyswood & Woodlander do for a N, shady foundation strip? We dug up some huge hosta clumps with plans to replace with shade tolerant Hs - also considering Little Honey for the W end of strip. My concern is the circumference, will they tolerate some pruning without sacrificing blooms? I've already started with MyM at the E end of the strip.
Appreciate feedback :-)

Hi, Dita.
'My concern is the circumference, will they tolerate some pruning without sacrificing blooms? '
If key word is 'some', answer will be 'yes'.
Consider them all 5x5 and plan/plant accordinghly.
Special note about MyM.
IMO it must be planted as a solitary plant with a close access to it to admire its subtle beauty when in bloom. On another side, unlike other abovementioned serratas it provide no specific attraction past bloom, so you have to chose planting spot very wisely.

Do you have a brand new LH or you planted it last year?
Affraid to jinx it, but seems like mine already settled in. It overwintered past winter on A- (minus for one dead secondary branch).

Best wishes.

Many thanks for your prompt & helpful input Ego45!
It looks like I may have to relocate MyM to my wider front strip where she can be admired closer ... indeed 'subtle beauty' & charm tho quite shorter bloom time for our admiration. She gave me a few samples on her first season and some without the fertile florettes - IMHO quite charming indeed! I may even relocate my '07 'Midnight Dutchess' in the shame site since I already have a few astilbe clumps there.

I think you may have a 'Jogasaki' - how did it overwinter in your zone! I'd like to look for a bit hardier serratas for the shaded foundation strip that will allow a bit of trimming. Since I plan to take MyM from the strip I'm looking for 3 specimens to replace the 3 huge hostas we dug up. There will be smaller hostas & astilbes there too.

I'll plan for LH, if I find one in town for another site. BTW did you protect yours ,,, how seriously, if you did!

Thanks again!!!

Ditas,
My Jogasaki is nothing to talk about yet.
I was given two cuttings last July by one of the very reputable member of this board. They rooted and spent whole winter in a cold frame. They are well alive and now due to be transplanted from 4" into 2G pots. They'll not see real soil till spring of 2009.
As to third serrata for your strip, I would strongly endorse Preziosa (for the appearance only, as hardiness in 5a is unknown factor to me). I think Preziosa flanked by Greyswood and Woodlander should make this combo work. It is of smaller statue than other two, mophead, not a lacecap and should make a nice separation between those two.
Re: LH.
No, I didn't winter protect it, except burlaping it for 3 nights on a row when we had low single digits. As soon as night's temps came back to our normal 10F+ burlap was removed. That's it.
Such a beauty! Even if you have no chance to see it ever blooming foliage alone worth all the troubles to protect it, which considering its size probably will be not that hard.
Here she is in last June. By the end of the season it was 2x2'. Blue foliage of Halcyon makes yellow LH even more stand out.

Hi Ego45 - You have the loveliest layouts & healthiest specimens! Indeed the LH surrounded by the blues & cream-centered hosta peeking in the upper corner - just lovely - as I find the tiny blue scillias & mini daffodils, stunning, welcoming Spring & announcing the even more stunning season of Hs ahead!

The wee-hours of the AM thunder-clapping-storm here, drove me to Dirr (H bible?!) in search for hardier lacecaps for my zone5 (not much help from this said bible!). Along with your suggestions, I also marked Blue Billow, Coerrulia Lace, Lanarth W, Veitchi & Frillibet (this one, you or Hayseedman suggested last season, to consider!). I have 2 other bright-shade/2hrs of late AM sun readied holes, to fill with these serrata beauties!

The N shaded strip is 3.5ft/wX26ft/lg has the 3 estab. hostas between the planned combo of serratas & some astilbe clumps too.

Just below the front porch is a strip, 5.5ft/wX21ft/lg-W facing w/2hrs of dappled noon sun - later, shaded by grown trees (here just one side of the front steps, we dug up 2 ES that produced only 2 smallish clusters. This would be a good spot for MyM for well deserved admiration! There are established astilbe, bleeding heart & columbine clumps around ... I was thinking of LH between MyM & (16 Candles) Clethra as I have 2 gold hostas at the foot of a birdbath farther down ... what do you think ( MyM is more persian pink)?

Many thanks again, your experiences on Hs are invaluable to me & I do take your 'caution' about the difference in our zones - not to worry, I winter protect accordingly ... I have been dealing with the looks of the front in the Winter since we took off the generic evergreen yews that looked just as awful w/o snow cover!

OMG, those privacy fences are gorgeous!!!!!!!!!!!!!!!!

i have an ugly chain link fence to hide, too. yuk. i cannot wait until my hydrangeas look like that! how did she get those mopheads to grow so tall and so many blooms???

have you considered climbing hydrangea? they can get 50' tall providing they have something to grow on. in deep shade under a norway maple in zone 6, i have climbing hydrangea, lady in red, lace cap, oak leaf, and mop heads.

correction, not oak leaf but rather big leaf.