Hi jcyuhn -
If you do track down your vibration, please post the results. My E420 has some odd vibrations in the front end at highway speeds too. On the surface one would think it was tire balance - but the E has been there through the original wheels, multiple balances, new wheels, new tires and new tires again. We can rule out balancing!!!
Over the past year the front wheel bearings have been replaced along with the center link and dampener. So, that leaves things like bushings as you noted.
Aside from the odd vibrations at various highway speeds, the car is very sensitive to changes in the pavement surface - changes direction easily, bumps & expansion joints take the car a couple degrees off course. Nothing too annoying, I subconsciouly correct and don't even think about it anymore. But then I drive some of my friends' newer cars (LS400, C240 and 540i) and get secretively jealous over how smooth their steering feels!!!
I used to live in Plano and the roads were much smoother. I moved to Seattle last year and roads are terrible, so this has been bugging me alittle more than it did. And I will confess, I changed the rims from the OEM 15" to 16". The current tires or Pirelli P3000s - so far I'm very happy with them (on par with Michelins for ride quality and road nise).