There's some discussion on this very subject over at Bottle of Blog, along with a couple of bits of additional circumstantial evidence from some of the commenters.

I'ver never believed his conversion story -- I've known a lot of drunks, and not one of them was able to quit drinking without serious help -- but I always just figured that, yeah, it SORT OF happened the way he said. That is, he drank a lot, but he wasn't really addicted to booze, he was just a guy who'd never been forced to grow up enough to rein in his undergrad appetites.

I still think that's the likeliest explanation, but I'm not wedded to it if additional evidence surfaces for any other scenario.

over at Bottle of Blog

Yeah, The Poor Man post I linked to was pretty much just a link to the BoB post, but since I arrived there via TPM, etc. And I totally agree with your assessment until somebody presents me with a better explanation.

Meh. Bush has looked a lot weirder in photos where he was unquestionably sober.

Weirder? Yes. But drunker?

As I said, I'm a bit of a lush myself, so glass houses and all. But it's a funny picture, all the same.

Who cares! Drunk, coked, sociopathic, dry drunk, whatever. Motivation and character insights be damned! He is a dangerous scumbag but nowhere near as bad as his puppet masters.