maybe in theory, but if you use your imagination I bet you can come up with several possible valid legal challenges that could be made long before the electoral college votes are counted.
For example, right now Obama gets Secret Service protection paid for by the taxpayers. President Bush, or the head of the Treasury Dept., could attempt to subpeona the birth certificate from Obama to prove he's a legal presidential candidate deserving of the protection.
Also, state election officials would have the right (I think) to have Obama prove his eligibility before they allowed his name on the ballot in their state.
I'm not saying that Obama coudn't fight these attempts and even win, but I think there are some valid legal questions to consider.
There's no such thing as a "legal" Presidential candidate. "Running for President" has no constitutional existence.
Also, state election officials would have the right (I think) to have Obama prove his eligibility before they allowed his name on the ballot in their state.
Obama won't be on the ballot in any state. US citizens will be running for the positions of electors for President and Vice President. If you could prove that an elector pledged to Obama did not meet the criteria for the position, you could take THAT to a court.
But before the electoral college meets, these candidates have no actual legal existence.