I've just confirmed that this problem is new to b52 - javaw does not properly close on exit and keeps the memory to itself. b51 does not have the same issue.I noticed my pc was slow the other day. Seems the javaw.exe process kept running even after exiting b52, and it had grown to over 100Mb.
I like the new options and fixes in the builds, and I always try the new stuff, but this time I will stick with b50, I can't risk the session failing or losing my work. I run it in a netbook, so I don't have a lot of room to maneuver if something doesn't work right. I'm sorry but I'll wait until it's been tested more.
Hoping for another emergency bugfix build. If not I'll just have to remind my players to make sure it's not running in their processes after they close the program.