Here's an informative thread on Phormium:

http://forums.gardenweb.com/forums/load/grasses/msg090951505044.html

Near the bottom of the thread, one of our regulars states the plant may be cut back to rejuvenate it.

WCG, only time will tell. Since I live not all that far from you, I'm going to assume your winter was similar to mine and the sharp, very cold temperatures we experienced right before Christmas combined with a very heavy snowfall, produced conditions that most phormiums would find unable to tolerate. I'd consider these at best marginally hardy plants for our area, given that every few years or so we will experience a winter cold spell that will do them in.

IF the roots are still viable (and that's a big "if"), then cutting back to about 4-6" in early April will remove the dead foliage and tidy up the plant. But it will take several seasons before the plant begins to resemble its former appearance.

If the root system was frozen and is dead, it is relatively easy to determine. Just tug firmly on a few of the interior fronds. If they come away easily, the plant is lost. They will also be dark and discolored at the base where they emerge from the ground. If there is still good color at the base, chances are the roots have survived, although you will still most likely have lost some of the more exterior foliage (it will pull away easily also), but that happens in even mild winters. If most the foliage is still firmly attached and with decent basal color, wait until April, then cut back.

I cut mine back last week and it is firmly in the ground.
I am hoping to put it in a container later this Spring.
Since we just bought this new home last August, I have no idea what is coming up...but want to move some things around and make the garden more my own.