Chris Rawson at TUAW:
After installing OS X Lion last week, one of the things I’ve noticed is that Safari tends to run away with enormous amounts of RAM over time. A subprocess called “Safari Web Content” will sometimes use over a gigabyte of RAM. Since my Mac maxes out at 4 GB, and since it’s a rare day that I’m only using Safari and nothing else, having so much of my system’s RAM gobbled up by one process was bogging everything down.
There’s debate in the comment thread about whether this is the way Safari is supposed to work- use available RAM to cache data for quick and easy page swiping.