I am seeing this too. It shows my "Active" RAM as half used after I exit Parallels. I startup my Mac, check my RAM. Lauch Parallels only. Let it run for a bit, then suspend it. Wait a bit and recheck my activity monitor, and my "Active RAM is still high, but no paticular app is using alot of RAM. When I suspend it actually increases the "Active" RAM.
MBP CD 2.0, 2GB, WinXP Pro SP2, 256MB