I did the current update for the iMac 2.8 Intel i7 and I can no longer preform applejack. I didn't pay much attention to the update, but I know it had something to do with display backlight from coming on to early.
Is AppleJack still installed? Perhaps the update removed some folders in the root user's home directory.
When you say you cannot perform AppleJack, does that mean, it won't run (program not found etc), or that you can run it and something goes wrong?
I'm having the same problem. What's happening is that the software update is preventing the back light from coming on earlier than it needs to, so when you attempt to start up to use applejack it's as if the screen isn't on. I assume that the computer goes into single user mode ( I assume that's what it is called when you want to run applejack) because after I hold down the keys the computer is on but doesn't load to the desktop, it's just black and I have to hold down the power button to restart the computer. Now that i think about it, I'll have to try it again and just carefully type reboot and see it the computer restarts...
Thanks for the detailed description you added. It sounds like this ticket should be titled "New iMac update blocks single user mode." So I'm changing the title to that. Please file this bug report with Apple. It's important that users be able to access single user mode.