Yesterday, I did a new install of 3.0.1 and was surprised that a clean install used nearly double the memory of my existing, fully-loaded 3.0 install (with over 20 plugins, 1,200 posts, and 1,500 - 2,000 unique visitors/day).
Today, I experimented with an old "dead" site. At WP 2.9.2, it used 21MB.
Installing 3.0.1 brought that to over 24 MB.
Something is wrong with 3.0.1, causing it to suck memory.
I'll be sticking with 3.0 on my live site until this issue is resolved. It is presently using about 13MB.