a) They want news about the iPad 3 to leak out
or:
b) It's faked, something which, as several posters here have demonstrated, is trivial to accomplish.
I think b) is the most likely answer.
If 6.0 is unvelied next week, then Apple is doing that on purpose, as they know Ars checks their logs for that kind of content. However, if they stick to 5.1, then we'll know it's some kind of prank, and Ars will probably think twice before publishing the same article next year
c) Of the few hundred employees with access to iPad 3s, at least one slipped up and visited public websites with it.
This is the part that excludes (c). If they didn't want to leak, they would've configured the prototype units to report themselves as normal iPad 2s.