Following on from my tweet the other day, I thought I'd publish this 'trick' here.
I have found my terminal gets really slow after about six months of use, and the trick to clear it up is to remove a shedload of log files which OS X seems to hoard. Now, if you're particular precious about your terminal history then don't do this, but if you'd prefer not to wait a second for your terminal to log in, then you can clean up the logs like so:
sudo rm -rf /private/var/log/asl/*.asl
sudo rm -rf without understanding what you're doing can be dangerous. If you don't understand it, don't mess with it. If you do, it's not my fault you broke your machine. Please read more about rm first.