That day will come...
250.126 requires green screws with a hex head for EGCs, or a green nut, or a green pressure wire connector. on wiring devices. 250.8 prohibits sheet metal screws for connecting grounding conductors to enclosures, but (you're right) doesn't say what
is required. 250.148 says "grounding screw that shall be used for no other purpose". There may be other advice scattered in the Code, and all this is based on the 2005 Code -- don't know what 2008 (or your local AHJ) says.
Nice long thread about this on Mike Holt's forum:
http://forums.mikeholt.com/showthread.php?t=76912
I used to use one of the clamp screws in a box (after removing the clamp) but a local Master Electrician who advises me said that wouldn't be acceptable to our AHJ. An electrician in Pittsburgh says his AJH requires the green screws.
Bottom line -- if you use the green screws (which, BTW, one of Mike Holt's posters said
weren't Listed) you'll pass. If you don't, you might not.