I had the same issue when I switched from ethernet to wifi.
After a RasPi reboot I had no connection via wifi.
I "solved" it like this:
1. connect via ethernet
2. set up wifi
3. shutdown
4. unplug ethernet
5. start with wifi dongle plugged in
6. wait a few minutes (!) to connect

Then you'll run into the next problem: Raspyfi is not reachable because MPD has not started correctly, please see my crossposting here:
http://www.raspyfi.com/forum/r.....n-failure/

You can connect via ssh though. After restarting MPD Raspyfi will be reachable via web client again.

Please let us know if you have a solution!

Cheers

And when I disconnect the ethernet cable, then my wifi stops working and I can't get to the pi.

Any idea?

utterly frustrated with this so far.. This looks like a great project, I've received a lot of help, but I can't get anything to work fine.

cheers!
S.