Not to rain on our many commenters and emailers parades, but AT&T's VoIP policy has nothing to do with either of those apps because:
Google Voice was rejected/is still under review by Apple, not by AT&T. All parties have clearly stated AT&T has played and is playing absolutely no roll in keeping Google Voice out of the App Store. That ball is still firmly in Apple's court, and it will take movement from them, and them alone, to bring it to the App Store when -- and if -- they decide to pull the trigger.
SlingPlayer sends your home TV video over 3G, and clearly has nothing to do with VoIP. AT&T has a distinct and separate policy forbidding the use of apps like SlingPlayer on their 3G network. Unless and until AT&T changes that second, specific policy as well, no SlingPlayer over 3G for you. (And seriously, given AT&T is dropping 30% of calls in NYC as well, does anyone think their network could survive even more traffic at this point?)
So there you go. You're getting VoIP from AT&T and Apple, no more, no less. Scratch that off the list, and if they've given an inch, and you hunger for a mile, keep the armor on and re-focus your battle on the next two (or more) targets.