[robg adds: I'll admit I can't really explain this hint, but I can say that I've noticed odd things about iChat's spell check during my use of the program. It seemed sometimes I had on the fly spell checking, sometimes I didn't (depending on who I was talking to?). After trying this hint on Friday, and watching all weekend, every window had spell checking. One possible area of confusion is that there's no checkmark on the "Check spelling as you type" menu item, so it's hard to tell if it's enabled without typing something in a chat window. Anyhow, if you've been having trouble with iChat spell checking, give this quick fix a try...]

