Ever since that initial freeze, it started happening to me more and more. It eventually got to the point where unplugging and replugging the keyboard no longer helped. I'd tried different USB ports, but the same thing happened on all of them with the same frequency. I tried different keyboards (but the same macally icemouse). Here's the type of errors I was getting in my console:

Feb 3 14:26:53 MyComputer kernel: USBF: 4592991.182 AppleUSBOHCI[0x222a000]::MakeDevice error setting address. err=0xe00002ed device=0x378e100 - releasing device

I eventually isolated my problems to the mouse itself. I assume it was not responding properly to USB queries from the computer and was currupting USB communication. I'd been discussing the issue on an email forum at my workplace and another person who was having the same problem mentioned that his mouse and keyboard had been damaged by a static discharge. So that lead me to try swapping out the mouse. Ever since I replaced the mouse almost two weeks ago, I have not had it freeze up or lose power. (Note that whenever this happened, the light on my optical mouse would not come on - which I'd interpretted as a power loss.)

So I think this is all due to a dying/faulty USB peripheral. The problem with the USB driver is that it can't seem to keep the problem from affecting other peripherals (perhaps connected through a common hub, but I can't confirm this). Does anyone know a way to reset the USB driver so that unplugging/replugging is not necessary to temporarily recover from this situation until the peripheral can be replaced?