I was watching the Nirvana music video collection on MTV last night using my Formac Studio/TV in the dock and talking to my friend Jon on AIM. I wished I could get the dock magnification higher, so I talked to him about it and after a while, he and I found the value in the dock plist. In ~/Library -> Preferences -> com.apple.dock, there is a key named "largesize" followed by <real>123</real>, where 123 is some value (probably not an integer). You want to change that integer and reboot the dock; 256 works great for me. My friend posted a screenshot of it at spymac.

[Editor's note: This works only if you have magnification enabled, and don't use a value greater than 512, or else the dock won't work at all. Also, if you use the dock prefs to change the magnification, you'll lose your super zoom.]