Despite what anyone says about Linux being an OS you "never have to reboot", the reality is far from that.

There are simply too many variables and too much software in constant flux. I am sure it will be something specific causing problems, I might even suggest the web browser (I always find Firefox on desktop Linux to gradually degrade if I leave it running for days). I think overall, the software the end user interacts with is never quite up for the challenge of being running 24/7. The basic OS itself is, as can be identified by Linux servers running for months, but once graphics enter the equation all bets are off it seems.

That said I doubt its half as bad as Windows Mobile. I never left anything running on my Xperia X1 as it was terrible even using it for very basic purposes. I expect the N900 to kick its *** big time.