i'd say you're definitely going to be fine for a while yet in terms of support, the community base and maemo project for the N800 are both really really strong, any problems or queeries you might have will generally get answered quite quickly if you're in the right place, like here!

in terms of new devices, it's a tough decision to make, here have been rumours of an N810, but nothing confirmed, it's just the nature of the beast sometimes, there's always going to be upgrades...