The providers, that something that I can recommend you because is very personnal, but for the devices, I can personally recommend you the Palm Treo 750, or the HTC Tilt or TyTN II.

Viper

Anything but T-Mobile is a start, T-Mobile's EDGE is simply too slow for acceptable video. As I recall, Verizon's "unlimited" data really isn't unlimited and when you're talking about Sling, you're talking about having to watch that ceiling. Personally, I use Sprint. It's more than fast enough for the mobile Slingplayer and the Touch phone works pretty well. There's also no ceiling on their unlimited data package.

The BlackJack II from AT&T fits your requirements. It has a full keyboard, it doesn't have the poor video performance of the current crop of HTC devices (Mogul / Touch), it's very fast on 3G (check your area for ATT 3G coverage), it is slim and "pocketable" (something my Verizon 6700 was not), and AT&T has a "soft cap" 5GB/month data limit, meaning they don't charge for usage above that, but might ask you to leave if you routinely go significantly above that limit. While Slinging to your phone can use lots of data, I find I very seldom get anywhere near the 5GB with my occasional usage pattern. My alternative suggestion would be the Touch (Sprint or Verizon, but Sprint has the cheapest and best data plan). It is a great form factor, but it does lack a keyboard. The Mogul is really big and is just a pain to carry...I carried the 6700 for 2 years and it got really old.

I just changed from an ATT 8125 to the Tilt. The reason I didn't go with the Blackjack is it doesnt have wifi, I like the idea if I am somewhere with wifi I can use it rather than the 3g. But, that being said I did want 3g you can watch a show on 3g using Edge was a bit tough but you could make it work.

Slingbox Pro Slinging Comcast Analog and HD via Comcast Internet using AirLink Powerline adapters; Slingbox Classic Slinging Sunddenlink Analog Cable and DirecTV HR21 via Sunddenlink Internet, using netgear powerline adapters.