That's a fuckin' GREAT idea. Only problem is that I can't drive.
That's part of the appeal of SF actually, the fact that it seems to be quite a walkable city (unlike say Los Angeles).
Looking through one of the guidebooks, the Tenderloin area seems quite an interesting (and relatively cheap) spot to base myself. It does say it's quite dicey though, especially at night. Anyone know what it's really like?