I'm guessing b4 is playable.
But I think against a better player, Black is guaranteed a better opening position. When Black opens 1. e5, what does White play to defend his pawn? 2. a3 has almost no bearing on the game, while 2. c3 takes away a good development square for the Knight.
Next Black gets to develop a piece, which is probably 2. Nf6, and now I think White is actually behind in tempo. It's almost as if Black went first. Does White play 3. Nf3? Black can now defend his pawn with Nc6, and follow it up with d5, and he now has a strong control of the center.
White's pawn is attacking c5 and a5, but those squares don't have any immediate impact on the game. Moreover, White spent TWO moves to set up this wasted attack. White isn't lost per se, it's just that against a better player, I think it's pretty obvious that Black is left with tempo and a better starting position from which to work towards the win. However, I'm sure if a grandmaster was feeling it, he could totally wreck me with 1. b4! :)

I've taken an interest in this opening and was wondering if anyone has experience with it (more than playing it once!).