There was a small outfit that had AR's in 7.62 x 39 which used AK mags. I believe Sask Gun Works was trying to get them in at one time. Those rifles had great initial reviews, but at least one gun blogger rescinded his good review because of subsequent Q.C. problems.
I can't recall the manufacturers name, (?hydra?) and don't think they are in business anymore.
It was a great idea/concept (an AR-15 in 762 using cheap, reliable AK mags). It just wasn't executed properly.
Edit: I just checked the RRA site--those 7.62 x 39 AR's are dead ringers with the ones described above. It'd be sweet to have one. I'd wonder how long the direct impingement gas tube would hold up (esp. the inside of the tube) with corrosive ammo, being so inaccessible to clean and dry.