Obviously hard drive capacity is limited, especially on laptops. Using FileVault, I find that every few weeks, I need to restart, as my free hard drive space has been depleted with the things that I have been doing, and obviously, this munches into the available hard drive space. Furthermore, when restarting, the FileVault box which says "Would you like to free up the space? Although this may take a while and cannot be stopped, it is reccommended you do this now" appears. This dialog only seems to appear at certain times, and not on every restart, but I may have found a way to trigger it so that any free space that is still being used by FileVault but which is actually empty can be freed up:
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070315032720120