From my research into buying one of these it seems the best you can get at present would be one of the following:
12-24mm Nikon
12-24mm Tokina (Much cheaper than the Nikon with very similar optical qualities) - This is the lens I have personally decided on, just have to find a good condition second-hand copy at a reasonable price now!
10-20mm Sigma (Very sharp but distorts a lot, depends on whether you want straight horizons etc.)
They are the main ones I would start reading up on. Obviously, if money is no object, their is the Nikon 14-24mm but this costs around £1000.
There's also the excellent Tokina 11-16 f/2.8 and the recently announced Nikon 10-24 f/3.5-4.5.
I use a Sigma 10-20 on my D300 (bought before the above were announced) and I'm very very happy with it. Apparently quality control may be an issue, so go to a dealers and try lenses until you're happy you've got a good sample or order online and play the Sale of Goods Act game. I just lucked out, I think.
The 10-24 is currently Bloody Expensive (tm). That's probably due to the pre-order demand. If you can wait 2 months, that price will drop, though not by a huge amount.