When another member pointed out that there is, in fact, an API for TWAIN 64-bit support, the thread was left with a "we'll look into it" from Adobe's rep almost two months ago. This is rather disappointing for such an expensive piece of software, especially one in which scanning in an integral part for many users. In the end, I went to my beloved, free XnView software to do my scanning, and the multi-step process of getting the scanned image into Photoshop.
Note that Photoshop CS4 64-bit version does support scanning via the WIA (Windows Image Acquisition) interface if your scanner is also supported by WIA, but this is often much more limited than scanning with TWAIN.
I wonder what other limitations exist in the 64-bit version of Photoshop CS4. Have you found any?