You can search the forums for more details on this, but its a known problem with Ant 1.5 and Eclipse 2.1. This was one of the driving reasons to have Ant run in its own external process for Eclipse 3.0. Aparently most of the leaks have been fixed in Ant 1.6.
There are links in the forums to many of the bug reports on the Eclipse site against Ant/Eclipse/Memory Leaks.