Ynot, did you get an answer in the other forums? I have the same question.

Maples forum thread: http://forums.gardenweb.com/discussions/4597952/japanese-maples-and-hostas?n=3

The Hosta forum had a different take :-) It depends on what your focus is.........if you feature hostas, then one is less inclined to plant them where anything will detract from them. If you feature Japanese maples, then you are less restrictive although you must be aware of the dangers involved in messing with a JM's root system.

Personally, I find Japanese maples good companion plants for a whole host of shade or partial shade lovers.......as long as they are planted simultaneously and do not need to be lifted and divided frequently (if ever).

Have a six year old JM with hostas planted outside of it's canopy. Was thinking of planting some hostas closer. Guess I will dig a hole and see if I encounter any JM roots. JM and hostas look great together. Many hosta lovers plant them around JMs. I am not a divider of hostas. Thanks!