UPDATE:

I installed iAtkos 5i again on a spare drive, with 10.5.5

The plugin verification worked with the iLok using the latest PACE iLok kernel driver package. This contains Leo and Snow kernel modules.

So it's something to do with the 10.6 build - either that or PACE have built extra checks into their 10.6 driver.

Can someone suggest which of these differences might be most likely to be causing my problem, and what to do about it:

bootloader: 10.6.2, Chameleon, 10.5.5, PC-EFI

That's a big change right there. Can I boot Snow with PC-EFI ? Not sure.

Possibly something in the smbios ? Or DSDT ?

A totally wild-eyed-guess is that the kernel module for PACE is detecting something it doesn't like in the kernel space of the 10.6.2 build and refusing to authenticate the plugins. A decrypter or disabler or something.

Once again, appreciate any help.