I just downloaded the Photoshop CS5 trial version, and tried pointing the Plug-ins Folder option in the Prefs file at my CS4 Plug-ins folder. while I didn't expect that this would work for the 64 bit version, it did work for the 32-bit version without having to reinstall a bunch of filters. So far, everything seems to work just fine by pointing to the CS4 Plug-Ins folder.
These are the filters I tried:
Focus Magic
NIK: DFine 2, Color Efex 3, Silver Efex, and Viveza 2.
Topaz: the latest versions of Adjust, Clean, Detail, Remask, Simplify, Denoise and DeJpeg.
Vertus: Fluid Mask 3.
No crashes, freezes strange cursors, etc. At least for the 32 bit version there is no need to reinstall the filters.