Front end vibration

Hello! I have a 2002 Buick Park Avenue with a 3.8 litre engine, with about 59,000 miles on it. The front brakes and rotors were replaced in April of 2007. Recently, the car has developed a front end vibration (driver's side I think) at all speeds. It does not increase or decrease with braking, but it does increase with speed and decrease when slowing. Today I had new tires put on as well as an alignment performed. The vibration continues. What could be the cause? Thank you for any help that you can provide!