Even if the next version of exchange is 64bit only its not even due to release 1H07, very few users will switch to it straight away, most will wait on SP1 which will probably be 1H08 so i reckon it'll be a couple of years before we see major 64bit uptake outside of the science sector.
Agree with you on the drivers though - they are the main problem. It seems like a catch 22 - oem's don't put much effort into x64 drivers because no-one is really using it and no one really uses it because the driver support is rubbish!