i have to say - after fiddling this for the last two hours (holy crap it's 3:30am), i think editing AGPM could actually be a temporary solution until Lion rolls around with their updated drivers. granted, this could also necessitate flashing your card's bios to increase the lowest two core/memory/voltage states, but here's where i'm at right now.
i've edited AGPM to effectively have only 3 power states for the GTX460 on an iMac 12,1. console is only showing the card switching between states 1 and 2, with the lowest (3) ignored except for a few seconds upon first boot. i've also increased the time between switching states, just in case the freeze is caused by too many rapid state changes.
for those using iMac12,1 in your SMBIOS with a GTX460, please try this AGPM.kext out and see if it works. you'll need to replace the one currently in your S/L/E. use something like Kext Utility b7 if you are unsure how to add it manually while repairing permissions. except for logging the state changes, there should be no other error messages in your console.