I don't see how that'd either be possible, or really matter to us for our purposes, because in our corner of the universe the speed of light does not vary, so to speculate about it's variance is pointless for our purposes.
Wimms, the thing is is that the speed of light in a vacuum just does not vary. It stays at c. Even if it varies, our measuring equipment would vary with it, and like Hurkyl(...I think) said, then we'd implement Occums razor and simply state that for all intents and purposes, c does not change...