This trick works for any program on the Mac which uses the default dictionary (the same one that TextEdit, Mail, and the other built-in programs use). So when I accidentally added a misspelled word to the dictionary in Google Chrome, I wanted to remove it. Here's how:
Type the misspelled word
Right-click it and click "Unlearn Spelling"
Done! Works across-the-board on Mac OS 10.5 (I'm pretty sure), and higher -- I am running Mountain Lion.