Insiders over at the AVS Forums - who are proper, actual insiders who work for companies like Microsoft, Universal and representatives of the Blu-ray consortium - reckon the decision could've gone either way.
Warner dumping HD DVD for Blu-ray went down to the wire - and it could've been persuaded, along with 20th Century Fox, to go exclusively with HD DVD instead.
In fact, they both nearly DID - an agreement was apparently in place between Warner, Fox and HD DVD backer Toshiba for the HD DVD WIN SCENARIO, only for Fox to pull out at the last minute and go crying off to Sony instead. Which gave Warner cold feet, so it went Blu-ray as well. It really was that close to being an HD DVD victory.
So if Warner and Fox had gone for HD DVD it'd be Blu-ray that'd look like the failed format today, and perhaps Bill Gates just might've pulled out an HD DVD-packing Xbox 360 from under his podium at CES last night, rather than blather on about a few new downloadable films instead.