Nikon Scan will run in OSX up to and including Snow Leopard. At least it does on my machine - MacPro 2 x 2.66 GHz Dual-Core Intel Xeon 8GB 667 MHz F8-DIMM. It will not run on Lion which is my main operating system. I have Snow Leopard installed on a separate drive with Nikon Scan, so have to reboot into that to scan.
Hope this helps.
Interesting and your fortunate, because this seems to be "touch and go". Here is Nikon's own statement about compatibility from the Nikon Scan 4.0.x download page:
Mac OS 9 (9.1 or later), Mac OS X (10.1.x, 10.2.x, 10.3.x, 10.4.11) (PowerPC Macs only, Intel Macs are not supported*).
NOTE: Mac OS 10.5.x and 10.6.x are not supported and not guaranteed to operate properly with Nikon Scan software. We suggest the use of third party software such as Vuescan or Silverfast with either of these operating systems.
*While complete support for Intel Macs has not been announced, we are able to run version 4.0.2 successfully on our test Intel-Mac systems without issues. Install 4.0.0 first, then the 4.0.2 update before running Nikon Scan. Please allow a slightly longer than usual install time.
I'm wondering whether operating the computer in 32-bit rather than 64-bit mode is what makes the difference?