I have a Samsung Captivate. It was using a horrible amount of data while still on my home network.

When the Captivate goes into sleep mode it drops your home network and reverts to 3G.

I found a solution on an internet forum. There is an Android application called "Keep Captivate Alive."

Once I installed the application my Captivate stayed on my home network even when the screen turns black.

AT&T was of no value in helping me solve my problem. I have to believe there are similar applications for other Android and IPhones.