If it worked under Panther, then it should work under Tiger. The difference? I'm not completely sure. There are several possibilities: iChat 3 uses different ports, or the Tiger firewall automatically blocks more/different ports. From my research (about 30 seconds with Google) it looks like both. Look at this MacOSHints page on the subject and see if the solution presented doesn't help.

It floors me that one part of OS X.4 would not "know" that a new program distributed with it used an "uncommon" port and thus open that port if iChat 3 was installed. Not good coordination, guys!