If you live up where it's cold, water/ice inside the tire is a possibility though I never thought of it before - if you put air in when it's hot, there's more water in the air. When it gets cold, you naturally would add more air to your tires, i.e., adding more water. In the cold the water would condense and possibly freeze into a lump.
With this condition, if you drive long enough I think the tires would warm up enough to melt the ice that's inside so the vibration would disappear - the water would spread out over the inside surface of the tire evenly so no unbalanced loading would occur (I'm theorizing here - ever do any spincoating in a lab?). Once you stop and park the car the water will refreeze into a lump in the cold.
One quick and easy way to test this would be to bring your tire into your home (should melt any ice inside the tire) and see if you can hear any sloshing. Of course you will have to identify which tire is causing the vibration.
What does everyone think?
Otherwise rebalance the tire/wheel. If the rim is bent you might be able to see it wiggling on the machine while it spins. If the wheel is bent, ouch. But get it repaired/fixed immediately before you damage your suspension and bearings...
TireRack has a good chart on diagnosing wheel/tire vibrations, you should check it out...
http://www.tirerack.com/images/tires/vibechart.pdf
