According to Kevin Purdy over at Lifehacker.com, Android 2.3 is getting a SIP client built in. He says:
If you've got a SIP account set up, or your friends and contacts have SIP numbers, you can now make calls to and from those digital numbers through the Android phone without special software. As you might guess, though, carriers will likely have some control over this option.If you don't have Android 2.3 you can always download 3CX's Android client. Although it's really hard to beat built in.
See Screenshots and more of Android 2.3 Here:
Okay, I'm not complaining here, but Windows Phone 7 team...you listening? ;-)