Debian is one of the oldest linux distributions and was already old and mature when N900 came. And it will be around for long after the N900, so no need to worry about that.

I also believe there will be some sort of communitysupport as long as the phone can survive mechanically. Those fashinpeople who cares about nokia support, upgrades and such stuff will ofcourse dissapear very fast. And leave behind a core of very intelligent and knowledgeable people, that thinks the N900 is unique.

Compare it to Nokias real communicators (Not e90) who had communitysupport for many years after they were dropped by nokia. And in some extent to commodore C64 and Amiga, who still has plenty of communitysupport despite the fact that they are homecomputers that havent been manufactured in 30years.

However, even if you were the last N900 user on earth, you could still get plenty of help from knowledgeable Debian users...