Receiving calls over Wi-Fi first I just got an android 2.2 prepaid phone (no data plan, so I'm planning to have 3G off). I would like to find some service/android app combination that would let me receive calls over Wi-Fi on Skype or Skype-like app if I'm connected; and then redirect to the phone itself if I'm not. I looked into Google Voice, but it doesn't appear that there is a way to set up an order preference...

Ideally, I'd like to be able to port my old phone number (from my service that I'm about to disconnect) to this service.

Does anything like this exist?
You might want to take a look at Asterisk, an open source PBX.  You can create a dialplan that will ring your Skype or sip address first, and then switch to your prepaid phone number. With version 1.8, Asterisk supports Google Voice in addition to Skype. Dialplans are completely configurable, however this does entail setting up a server to run Asterisk. The good news is that the minimum hardware requirements are very low; you can even run it on a consumer router running custom firmware such as DD-WRT, OpenWRT, and Tomato.

A couple places to get you started:


Asterisk: The Definitive Guide - a free book Oreilly book, written for an older version, but still a great primer
Asterisk 1.8 documentation