After upgrading to Snow Leopard, I found that I can change the icon size in the Finder's Icon view windows using the MacBook's multi-touch trackpad. To do so, just use a two-finger zoom/pinch gesture to either zoom in or out -- this works on the Desktop, or in an active Finder window in Icon view mode.
[robg adds: I was hoping this might also work to change the text size in List and Column view modes, but no luck. It only works in Icon view mode.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090902234027381