IMO, unless there is something you absolutely, positively, can't live without (or with, in the case of your cracked screen) on the 3GS, hold out for the summer, when Apple seems to like refreshing the iPhone.
Summer 2007: start selling original iPhone
Summer 2008: update phone to 3G, add GPS, more memory, etc.
Summer 2009: Update to 3GS: add voice recognition, compass, better camera, etc.
Summer 2010: Update(?) to ???: Who knows what they'll add/change/improve this time!
I got a 3G about a month after they started selling it (July 2008). I think I'm eligible for a 3GS now. There are a couple things I'd like that the 3GS has that the 3G doesn't (voice recognition & compass come to mind...), but I figure since I waited until the 3G came out (never owned an iPhone "Classic"), and suffered with "old school" phones that year, I can live with the second-best phone there is (the 3GS being the best, of course!

).
Besides, when they do update the iPhone next, wouldn't you rather be able to grab one straight away instead of waiting another year or more? I know I would!

