Checked the crash logs? Can be tricky to decifer, but a few minutes picking through it, will more often than not give you a clue, if not the answer to what is making ichat crash.
In my experience, it is nearly always 3rd party ichat add-ons that make it crash, especially if the add-ons are outdated. iChat usbcam was good for doing this..
Failing that, trash the prefs.
Hey, can you hear that? I think its a raging stampede of die hard Adium users preparing to stake their case..