Not only can I use Slingplayer via 3G, any other WiFi required iPhone application I now can use via 3G.
The key is not to use AT&T, but to use Verizon. But how to use Verizon on the iPhone? Well, you must have a Verizon data account, then you will need the Verizon MiFi. The Verizon MiFi takes your 3G and turns it into WiFi. You just then connect the iPhone to WiFi.
The MiFi is very small, about the size of a credit card.
I just received my Verizon MiFi 2200, plugged into my laptop and connected. Then unconnected the MiFi from laptop and pluged into AC power. Once connected to the Verizon network, you can unplug the MiFi since it has a battery so you can then unplug and move around. The MiFi only needs to be connected to a computer to initially access the network. It then runs as a secure WPA wifi network.
I ran the speedtest on my Macbook and am getting 1294 kbps down and 487 kbps. I also ran the test on my iphone and got 762 (876 max) kbps down and 194 (211 max) kbps up.
My next test was how far away from the MiFi would my iPhone still be connected. It was about 50ft from my office that I loose connection. That is through 2 doors and into the lobby of my office. Much further than expected.
So far I am completely impressed with the MiFi, it works as advertised, and would highly recommend.