The 
Nokia N900 comes with a 
root filesystem with a minuscule 256MB.  
Nokia and the 
Maemo community have done a good job of making Maemo work with this through the liberal use of 
symlinks, but a limitation is still a limitation, and Nokia's still trying to get it right.
This morning I had about 27MB of free space, or about 10%, free.  This hasn't been a big problem before, but today the first 
Maemo update after I received my device was released, and it need about 14MB to hold the downloaded files, and would need about 17MB of space extra after extraction.  After combing my 
filesystem, I realized that I had over 6MB of 
Maemo themes installed to the root filesystem. 
After cleaning that up, I had 33MB free, which I would have thought were enough, but unfortunately, it wasn't for the 
Maemo application manager.  A solution that seemed to work was to do it the 
old fashioned way and pull up an 
xterm window and run 
sudo apt-get dist-upgrade manually.  That seemed to work just fine.