This is a new one as Ben says. This is an issue with the new update for android but no real work around I have found so far I'm afraid. All I can recommend at the moment is strengthening the wireless signal you connect to. Here's a guide if you need it. Changing the channel and mode may help until a more permanent fix is available.
I have same problem. If you go in settings, tap wi/fi, tap menu key, select advanced and then check automatic network switch it stops it. I've checked and so far it hasn't used any mobile data, just stops the annoying pop up.