Just popping in to say, the game is dated now: June 2011. So, not too long of a wait.
The two, million-dollar questions now are: how much, and who is going to be offering the game for DL?
On #1: $5 - $20, as they've stated previously, and nothing firmer. My guess is it will be $20 or MORE (although probably not more than $25) because interest in Terraria is through the roof. I expect them at the very least to max out their original asking price.
On #2: Lots of rumors. Valve contacted them in April, and the team has been in meetings since then. They won't disclose who they're talking to. One of the artists said "it's definitely not Steam", but I don't put much stock in what a new hire who shouldn't be talking about that stuff really knows. I think a Steam release is still highly likely (and is one of the biggest UH DDDEEERRRRRRRR moments they could mess up if they don't sign a deal with them), but others are potentially Impulse, Desura or some other DD alternative. They haven't commented on whether they will offer the game for DL independent of vendors.
So here's to hoping, at the very least, to no exclusivity arrangements on release. I'd much rather buy it off Steam than get it through Impulse, and I've only recently heard of Desura and am NOT in the mood to download yet another client for someone's DD platform. I've got at least 3 of those things already installed, and about half a dozen launcher apps from EA to Ubisoft. Gettin' tired of it.