I felt that the default text sizes in Safari were too large for my liking, but reducing them in the Prefs resulted in quite a few sites showing small, unreadable, and non antialiased fonts in places. In an attempt to fix this, I looked into the preferences for OmniWeb and copied the following lines into Safari's plist (~/Library -> Preferences -> com.apple.Safari.plist):
<key>AppleAntiAliasingThreshold</key>
<integer>2</integer>
<key>AppleScreenAdvanceSizeThreshold</key>
<integer>2</integer>
<key>AppleSmoothFontsSizeThreshold</key>
<integer>2</integer>
This appears to have fixed the problem, forcing all fonts to be antialiased (I know some people don't like it, but I do!).
They may not all be necessary, but I haven't tested them one-by-one to see.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030625004356264