The new set of drivers (which are pushed by Apple's Software Update package to Snow Leopard systems) fails to register correctly.

The installer hangs and generates an error message:

System extension cannot be used

The system extension "/System/Library/Extensions/hpPlugInInit.kext" was installed improperly and cannot be used. Please try reinstalling it, or contact the product's vendor for an update.

According to Apple support, this is a problem with the HP driver installation routine and not a Snow Leopard problem. They refer the caller to HP support.

Things that DO NOT work to correct the problem: downloading the drivers directly from Apple site, booting from the Snow Leopard installation disk, fixing disk permissions, repairing the main disk, deleting and reloading the drivers.

I have been unable to contact HP support about the problem; the HP website has no means for submitting a trouble report with driver installation.