Today, after some tinkering, I accomplished something of which I've long dreamt: placing a Voice-Over-IP call, from a real phone number to a real phone number, from my Android phone, using only my 3G/EDGE data connection. No plan minutes or hosted PBX phone service involved.
Here's a brief how-to (that's not even dependent upon having an Android phone):
1) Get Google Voice. (This step is going to be kind of a buzzkill for most people, as Google is still in some sort of indeterminate closed beta with the Google Voice system. I'm not exactly sure how I ended up with access, so I don't know what to tell you to do to get it too.)
2) Create a free Gizmo account.
3) In your Google Voice settings, add the 'SIP' address that Gizmo gives you to your Google Voice account, selecting 'Gizmo' as the type of number. (Detailed instructions.)
4) Install a Gizmo-compatible client on your phone. The folks at Gizmo have written clients for many popular phones. You can get one at http://gizmo5.com/pc/products/mobile/. If your phone supports 'J2ME,' then chances are they've got you covered.
(On Android, I installed 'sipdroid,' which isn't a Gizmo-specific application, but one that can handle any Voice-Over-IP service. (You can find sipdroid in the Android Market, but that version only works via wi-fi. Get the full version which supports 3G/EDGE calling via their site.) Configure it using the info from the Gizmo support page.)
5) Use Google Voice's web interface to tell it to call whatever number you want, selecting your Gizmo number as the one to ring when connecting. (Gizmo offers incoming calls for free; telling Google Voice to initiate a call and ring your Gizmo is technically an incoming call, even when you're calling a friend.)
6) Tell the Gizmo client on your phone to answer the call. You're now connected, and you're not using any minutes.
A nice side-effect of this Gizmo compatibility is that you can run Gizmo clients on any computers you have around. When someone calls your Google Voice number, all the computers will 'ring' as well as your cellphone, so you can answer it from one of them instead. More minutes saved.
UPDATE: If you only want to place/receive calls from your computer, here's how to do it from GMail/Google Talk instead of Gizmo.