After updating to Snow Leopard, I've been having issues with accidental input on my trackpad. I had forgotten where the checkbox was to disable the trackpad when I had a mouse hooked up, so I popped into the Snow Leopard help system to find the solution.
Unfortunately, there wasn't an answer there; the only thing I found discussed the "accidental input" option:
To make your trackpad inactive: Choose Apple menu > System Preferences, and then click Trackpad. Open Trackpad preferences. Select the "Ignore accidental trackpad input" checkbox to make the computer ignore the trackpad while you type.After some digging, I discovered that the checkbox has moved to the Universal Access System Preferences panel, under the Keyboard and Mouse tab. Now if I could only find out why it was moved there.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20090911081118487