The player says he thinks he unlocked the outfit by killing a R* developer online (on the Xbox 360 you unlock an achievement for doing this, so it's possible this may be a PS3 exclusive). You can see some lovely photos of this over at Kotaku.
There are a few comments indicating gamers have seen zombies in online play, but nothing about transforming into one yet. Theories range from getting killed by a Rockstar employee, to killing a Rockstar employee, to getting the highest ranking.
They quote the official strategy guide (I don't have it) "If you see unclothed characters, look out! Only players who have reached the max ranking of 10 and a few other "special" people get the honor of shooting in their skivvies." But this player says he's only at Level 2. The user's name in the forums is GTA. I couldn't find a PSN ID for him.
Let us know if you've come across this by posting in our forums.
Source: GameTrailers thread and blog post