He's literally knocked over double the number of righties as lefties with half of those caught behind, though. If anything, it'll probably be the lefties who might eventually get on top of him because he's generally beaten them with pace rather than, say, movement away.
Cricinfo Statsguru - MG Johnson - Test matches - Bowling analysis
There's a lot of mythology associated with Johnson's bowling at Test level and that he's easier to face if you're right-handed just because he bowls a lot of wide ones outside off is part of it. I mean, seriously, when he's taking wickets against the top two batting line-ups in the world right now in losing series', eventually those who claim it's all luck, etc. will have to start revising their opinions or they're maintaining it in the face of some pretty strong evidence to the contrary. Sure he bowls a few more wide ones than most. But geez, he also bowls a lot of balls in some pretty awkward areas too. And for a bowler of that pace, he's remarkably economical too.