mail for exchange is listed in the official specifications and should take care of synchronisation with an exchange environment.
this software is avaliable for s60 for quite some time now, and it works great, i use it constantly.
so no need for active sync or pc suite.

i didn't really miss subfolder support, because when i'm going somewhere only the most recent messages are of interest anyways.
but since the n900 is marketed as an always-on mobile computer maybe they improved mail for exchange as well.

the version of modest for earlier tablets did support multiple folders, so i think it's not too far fetched to assume the maemo5 version will do so too

EDIT:
one thing to keep in mind is of course the fact that you need a properly set up server to be able to sync with. a working outlook client alone is not a viable sync target for mail for exchange.