well, I'm stumped. wheel balance is the usual cause for vibration at high speeds. and you've taken care of alignment as well...

Might be a flat-spot on one of the tires from slamming the brakes. sounds far-fetched,especially on a car with ABS, but i had a slight flat-spot on an ABS equipped '92 S600 that would only become noticable at higher speeds.

Where do you feel the vibration? through the entire car overall, or is it more pronounced through the steering wheel?