There are two "connectable" USB ports connected to the high speed buses on most MBPs. One, almost invariably, also powers the built in iSight Camera. In later models you may find that the other also powers the SD slot. You have to factor the demand from such devices into your calculations when considering what they can carry. In your case, woof, it sounds as if your iSight may be defective and drawing more juice than it should.
It is a real pity that more people don't acknowledge the value of FW as a far more competent alternative. If you ever have a choice of using a device over FW rather than USB always choose the former. More power without probs, more speed, less impact on your CPU to keep it running. You name it. USB, sadly, is pretty ordinary technology.
Cheers
Rod