I just bought a 2000 Impreza RS with the stock size tires 205/55/16, Pirelli P6 on the stock RS rims. When I bought the car it had a vibration from about 68mph-80mph. I had the tires balanced and the weights where way off. For the next two days the car drove beautiful, no vibrations, nothing. Then driving to work this morning I had a vibration right about 75mph and the car was pulling right. The tires look ok, rims don't "look" bent (they have some minor curb rash) and I didn't hit any curbs/pot-holes since the balancing. The car has 43,xxx miles on it and it could very well need an alignment but the vibration is puzzling me, why would it be fine for two days and then all of the sudden start again (from what I can tell I didn't loose any wheel weights, each wheel still has 1 clip on the outside).

maybe some weights flew off the wheel...

check to make sure your lugnuts haven't partially loosened, that can be a factor a few days after mounting ..

I'd check the wheel weights also. When I first had mine re-balanced, drove nice for a couple of days, then a lot of vibration. Threw off the inside weights. Something about the wheel design that the weights wouldn't stay on. Finally used tape on weights for the inside of the wheel. Cured the problem. If yours is pulling all of a sudden, take it back and have it re-checked. Also check your tire pressures. Lug nut check also. Make sure they torqued your wheels to the right specs. Warped wheels from overtight lugnuts are not unusuall.

Thanks for the suggestions! I'll be taking the wheels off when I get home and look for the obvious signs of missing wheel weights on the inside, re-torque the lugs to the correct spec (88nm I think) and check the tire pressure.