Thanks for confirming that I'm not insane! The first time I tried to set this up, that's what I thought, too. Later on I realised that I was wrong, and I thought I must have been insane to have ever thought it was limited to just those two domains.. Since then, I've set up all three of my google-apps domains..

The secret is: Don't click on the arrow to reveal the drop down list. Instead, click on the bit that says gmail.com - it's actually a text area, and you can edit it to say whatever you'd like. In other words, just type your own domain in there