After some tinkering i was able to get my Scansnap S510 to work under the windows 7 RC x64 and thought it would be helpful to share some results.
First, as you point out, you need the ScanSnap Vista x64 drivers .... available with some pleading from Fujitsu tech support.
After installing Scansnap Manager, I looked at the device properties, and it seems the problem is linked to the fact that the Vista drivers are not authenticode signed (seriously Fujitsu, is it amateur hour over there?)
I took two approaches, some combination of which worked for me...
1) I used a tool called the Driver Signature Enforcement overrider, from a site called ngohq.com that puts Win7 into 'test mode'
2) In the C:\Program Files (x86)\PFU\ScanSnap\Driver\SsDrvRepair folder, there is a tool called SsDrvRepair-x64.exe, that I ran. It pops up a big red warning against installing unsigned drivers, i install anyway.
And it works!
Not sure if I had to do step 1 or not. but it seems to me that the Vista x64 drivers would have been fine if they were signed by Fujitsu.
Good luck