Why is Macbook's USB only working at USB 1.1 speed?

The front USB port on my black Macbook no longer works. The rear port only works at 1.1 instead of 2.0 speed. I'm not entirely sure when the problems started. But I can think of two points of trauma the machine has suffered. Over a year ago, I spilled some distilled water into the back of the machine. But after it dried out, it booted up without a problem. I don't recall any USB troubles at that point.

Most recently, I replaced the hard drive, but accidentally re-inserted the caddy upsidedown. I believe the caddy scraped something along the top of the inside of the case. But I reinserted it properly and the machine booted. The USB trouble may have started around the time of this incident, but I'm not absolutely certain -- it could have been before.

The USB problems definitely seem to be hardware based -- they're there when I'm booting up from the internal drive or an external drive and in Leopard and Snow Leopard.

I'm curious if anyone knows what kinds of specific physical damage might cause this kind of problem and if there's any hope of a repair short of replacing the motherboard. I'd be particularly grateful if anyone could share details about how the USB ports are connected to the motherboard and what enables them to work at 2.0 rather than 1.1 speed.