[DELurker]

As others have said, the pics look really good. I have some suggestions (don't we all?), but they're more in the vein of improvements or decisions rather than criticisms, however they may sound.

1) It looks like you're using 1x1x1/8 wall tubing. You may want to look at using 1x1x1/16 wall instead. Half the weight, and the bumpers should be taking most of the impact load anyway.
2) You may want to bring the wheels inboard of the outer frame elements. This would protect the wheels, reduce torquing/twisting through the frame, and make mounting the bumpers easier/more solid.
3) The comments others have made about the dual pulley systems are spot on. If you need to attach a second pulley and don't have enough shaft, you could bolt the second pulley to the first with two bolts.
4) You may want to consider a means of tensioning the timing belts you are using. There's nothing more frustrating than mounting everything up and then finding out that the pulleys are too far apart for the belt to fit over the flanges.
5) Since you have the time right now, see if you can design up or find a common design that will allow you to demount and remount the timing belts without doing anything other than detensioning the belts. Although FRC is not as brutal on timing belts as industrial machinery, I've seen too many oddball thing happen to discount the possibility.
6) You may want to change how the shafts are set up. Rather than putting shaft holes through your frame structure, try mounting a block off the frame that accepts the shaft end.
7) I personally am a big fan of bronze bushings over ball bearings in FRC. I've never had a chance to use them in a drivetrain and am not sure I would want to, but if you used them, your shafts can sit in the bushings and the bushings can be Loctited (641) into place.