This just in from Fujitsu:
We can now FINALLY upgrade.
Hello ScanSnap Snow Leopard subscriber,
The ScanSnap Manager compatibility update for Snow Leopard with ScanSnap S1500M, S300M, S510M, and S500M is now posted! This update is for compatibility with Snow Leopard only. Proceed to the following site for download and installation instructions:
http://www.fujitsu.com/us/services/computing/peripherals/scanners/support/sl_download.html
Best regards,
Fujitsu Imaging Group
ScanSnap Sales
We can now FINALLY upgrade.