I would absolutely LOVE a 12-24 f/2.8 L for wide-field astrophotography! Imagine the length of exposures you could get, or at lower ISOs, with a 12mm f/2.8 lens! Ooooh, the bliss! I'd spend the money for it, too...12mm f/2.8 astrophotography...man I'm DROOLIN!!

In terms of exposure time, here is what I figure. Currently, with my 16-35 f/2.8 L, I usually get about 30 seconds at 16mm out of it, at ISO 800 - 1600, for a decent "printable" shot (i.e. a shot that could be printed at native size...13x19 for the 7D...without particularly noticeable startrailing. Rule of 600 would indicate 38 seconds, so I shorten that a bit for printability). For a web-sized shot, I can usually expose for about 40-45 seconds, and often use a higher ISO. With the 12-24 f/2.8 L, I figure I could get 45-50 seconds out of it for printables, and maybe as much as 65-75 seconds for web-sized shots! And that is nothing to say of the wider field of view, which would be nice at times...