There is a principle of law, which has been recognized by the Supreme Court, called the De Facto Officer Doctrine:
"The de facto officer doctrine confers validity upon acts performed by a person acting under the color of official title even though it is later discovered that the legality of that person's appointment or election to office is deficient." - link
Thus, even if Obama is conclusively found to be ineligible to hold the office of POTUS, this will not automatically negate any of the decisions (including bill signatures) he has made.
de facto officer doctrine will not apply to Obama, he was challenged on eligibility before he became President and those challenges continue. He and the Nation were put on notice and failed to remedy the situation. It is also not just an administrative issue,
Brilliant. There are any number of means to avoid the magic “do over” button these birthers seem to cling to as their hope. I just want to wake them up from their day dreams so we can get back to actually attempting to prevent the end or if nothing else, slow it down enough to plan for it wisely.