if you have a mac and want to talk using your gmail name (why would you anyway, just use AIM like the rest of the population) either use iChat or download the free program AdiumX. Adium can connect to your yahoo, google, AIM, and other names through one simple program. You can also be on multiple names at once so you don't have to be fumbling back and forth between programs.
just a little FYI for those who cared