Look for uneven wear. Particularly cupping
http://www.procarcare.com/includes/cont ... ewear.html.
Tire cupping is one of the more common culprits of speed-related humming sounds and road vibrations. Unless it's really bad, it's often difficult to spot with an untrained eye. The easiest way to tell is to either rotate your tires or put on the spare.
The way to check for a bad hub (wheel bearing) is to jack the wheel up and try to move it by pushing/pulling the top and bottom of the tire. It shouldn't move at all. You can also buy a mechanic's stethoscope and listen for grinding in the bearing center as you slowly turn the wheel. I'd bet on a bad tire, though.
If you do have a link unevenly worn tire, keep in mind that something in the suspension is causing it to wear unevenly. Before you throw a new set of rubber on there, it'd be worth a once-over by a mechanic. Might be time for a new set of shocks, bushings, or just an alignment.
Let us know what you find.