The Twain Version 2.0 scanner specification was released on November 17, 2008.  If you assume about 6 months for scanner manufacturers to update their scanners and test and release the software, one would not expect stable Windows 64 bit drivers to appear in the market until about May 2009.

You can probably assume that any scanner manufactured prior to 2009 will probably have issues with Windows 64 bit.  Reports on forums are that the scan drivers will not install.

If the scan drivers install, the will probably work with Imaging Assistant because Imaging Assistant is 64 bit compatible.  However, as of this writing, April 2009, we have not found any 64 bit scanner drivers that work.

Scanner manufacturers have virtually no motivation to update old drivers to 64 bit.  They want to sell you a new scanner.

Imaging Assistant works well with 64 bit operating systems.  Most scan drivers are actually 32 bit drivers, but they run on 64 bit systems or 32 bit systems.