Slingplayer Mobile [$29.99 - iTunes link] for iPhone has just hit version 1.2 and the single-line update says it all -- 3G streaming enabled.
And it seems to be in App Stores outside the US and beyond AT&T. (If it's not in your App Store, let us know.)
It was a long time and a lot of controversy coming, but it's here, so go get it and let us know how it's working for you!
[Thanks to Scott for the head's up!]