I'm not really sure why this is an issue at all. Yes, "rogue" repositories could claim that you'd authored various commits. But, why would people be looking at these repos in the first place?

There's a very strong builtin trust mechanism on GitHub — mainly due to the fact that repositories hang off of "users" instead of them hanging off of "projects".

And, myself, when browsing around aimlessly on GitHub, tend to check out either reputable projects (where such behaviour is not likely to go unnoticed) or the repos of a hacker i'm interested in — who surely doesn't have much reason to fake their own commits? ;p

Heh, maybe this could be the basis for a GitHub Reality TV show. "Tonight: DHH fakes his own death!" ;p