Hey, guys - I have since found a setup that works well with SipDroid - it involves setting up a free account at pbxes.org, then setting up a trunk to my VOIPo account. After researching SipDroid on the internet, it seems that it works best with pbxes. If you're interested in giving it a try, let me know, and I'll do a write-up on how to set it up! I can have both my ATA and SipDroid connected at the same time with no problems, and the call quality is great.

BTW, the android device I'm using isn't a phone - it's an Archos 5 Internet Tablet. It's cool to be able to make calls in a McD's wifi hotspot!