I d/loaded the firmware update last night, restarted - and the computer took about 6 mins to restart.
'OK' I thought as it was updating the firmware it would probably take a while... When it restarted I got a BASIC sounding chime, followed by the normal chime, apple logo, apple logo with rotating flash clock animation, then the blue screen, finally followed by the OS & my wallpaper. The computer stayed at the blue screen stage for 5 mins though, and then some horizontal bars appeared at the bottom of the screen, the left most one was white with the others being grey. Then the computer booted up & I was welcomed with a message saying "Firmware update completed".
I thought my problems were over.
This morning I went to restart the computer, except it was taking an unusually long time to reboot. I timed it and it took another 6 mins to reboot - almost the same way as last night but without the first loud beep before the usual chime, and when it loaded the operating system it didn't say 'firmware updated' - but it still had those bars at the bottom of the blue screen stage.
Needless to say I wasn't impressed by this long boot time, so as soon as it had loaded I restarted the machine to see if it would happen again. It took another 6 mins to reboot.
I then got fed up and turned it off. Later today I turned it on and tried to boot up in bootcamp by pressing option + O - and the computer ignored my request and went straight to the apple logo...except this time there was no flash circle animation...I waited...waited for 3 minutes - still no circle, no anything. In fact the machine is stuck at the apple logo and won't get past that.
I've rebooted, turned the computer off and on again, and even sat there patiently waiting for it to slowly boot up - but now I can't get past the apple logo screen.
I've tried rebooting now about 20 times, it's currently been 14 minutes at the apple logo on the latest attempt - I think it's safe to say the firmware update has FUBAR'd my imac.

dunno how I'm going to recover my data I can't even get into OSX!!