Well, this is a pleasant surprise: Age of Empires II: HD Edition is live on Steam for anyone who pre-ordered—apparently it has been as of the 5th. The store page still says it won't unlock for about another 17 hours, but the game launches fine, and all modes seem to be available. Since buying it now is still technically a pre-order, you should just be able to hand over your e-cash and get immediate access. It's essentially a soft release.
One notable feature not mentioned before is a new option in Skirmish mode to set the food cap to 500, though it warns that a faster computer may be required. So, in addition to a nicer interface and high resolution support, you should be able to make enough elephants to fill an entire 1920x1080 display and utterly trample your enemies. Such shenanigans are the lifeblood of the series, after all. We'll have more on the HD edition later this week once we've finished sinking into its nostalgic depths and wondering where large portions of our life went.