[Ether]

Al is correct about the 1 atm thing, but only in a painfully pedantic way. It does take a tiny (and I do mean tiny, neglecting any fill valve characteristics) pressure differential to cause gas to flow into an uninflated flat Mylar balloon... because the pressure inside the balloon must lift the "top surface" of the mylar as the balloon changes shape from flat to an oblate spheroid. In an earlier post I gave an estimate of the theoretical pressure differential required to do this.