I just realized that by disabling all unnecessary plugins (which for me means everything except the Search plugin) in Reader 6 (in Finder, Get Info on Reader 5 and select Plug-ins), I could have tremendous speed and memory gains at startup. After disabling the extra plugins, Reader 6 dropped from 14 bounces to launch and 5.6% memory usage to six bounces and 3% memory usage. It now is a fast and lean as Acrobat 5.
PS: this might be useful in other applications, for example I do it in Spell Catcher to disable unneeded dictionaries.
[robg adds: I haven't upgraded to Reader 6 yet, so I haven't verified these claims.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030712203449472