If you live in Europe, you can get the x64 ScanSnap software if you provide the info from this Fujitsu page:
Fujitsu | ScanSnap S510 | FAQs
(The IP is registered to Fujitsu Europe, and I verified that with Fujitsu themselves).
Unfortunately, Fujitsu Computer Products of America said that those drivers were "European only" due to "different hardware", that I could be put on a mailing list and x64 drivers should be out "by the end of this [2008] year".
If anyone from Europe (or outside) wants to try to get these drivers and test them, I'd be curious to see if they work with non-European hardware and how well they work overall.