While I personally think foreign-born Ted Cruz is eligible because he was a US citizen at birth, there is certainly an argument to be made the other way. It will be interesting to see how strong the opposition to Cruz is compared with the parentage-based opposition to Obama.
If Cruz is a serious candidate, it would certainly be possible for a legitimate challenger for the Republican presidential nomination to bring a lawsuit (even if it were a friendly suit) claiming that Cruz was ineligible, resulting in the adjudication of the definition of “natural born citizen” by the Supreme Court. I think that would be a good thing.
The birthers have so polluted the discussion, and popular opinion is so terribly confused, something, I think, is needed to clear the air.