AT&T has made sure that Sling Player doesnt work on 3G or EDGE simply because it doesnt want to choke its data bandwidth. But if you are on a jailbroken iPhone, you can easily change this settings in sling player and watch video over 3G.

Here is how to do it:

1. Open Cydia app from your jailbroken iPhone
2. Search for Tricker ThreeG app
3. Tap Confirm and Install Tricker ThreeG
4. You can now run Sling Player on 3G and EDGE.

Note that this trick works for other VOIP programs as well. “Tricker ThreeG” app uses MobileSubstrate Library  that tricks apps into thinking that they are on WiFi even though they are on 3G/EDGE/GPRS. In the default settings you can use Sling Player, Fring, Truphone and Skype over 3G or EDGE.