This is more likely to be a hardware caching issue than a memory leak problem.
See here: http://www.linuxquestions.org/questi...hreadid=253226
or search the forums for the word "swappiness".
Basically, your system is only swapping out pages that it has in memory but isn't using (eg. for a daemon service that never gets called), in order to use more RAM for hardware caching. There is no performance penalty for this, except a possible latency issue for re-loading the swapped pages. You can stop it from swapping data out to make room for caching (with the swappiness file) if you like.