We can reveal that the actor who will be voicing San Andreas' Carl Johnson is Young Maylay, he's an up and coming rapper from Los Angeles.
Dan Houser described him as having a "powerful" voice. Official Playstation 2 Magazine USA, Issue 86 was the mag that had the info, an extract from it is below:
"Actually, it was through DJ Pooh. He's been a producer forever...I was introduced, then the Rockstar family picked me up...I didn't know that Dan was listening, and I guess he was getting the idea about me being the voice right then."
Thanks to GTAGaming for the heads up