Credit card in the superdrive will get the disk out. I have the same problem with my 20" 2007 iMac. Chime, white screen, all drives spinning and powered, then nothing. The only keyboard shortcut that appears to be working is PRAM reset. I tried all of the above tricks and every other one I have been able to find. External monitors, known good RAM, new drive, new disks, 3 keyboards, target mode, and many, many more.
I finally gave up and took it to the Apple store. Three weeks later they told me it was the logic board. $435. I called Apple, emailed Tim Cook, dealt with Apple executive relations. They finally waived the labor at the store - $39 discount. woo-hoo. They refused to tell me the failure rate for this component or what rate qualifies as a known issue.
I'm looking for a "soldering iron" type solution now. I refuse to pay Apple's part-swappers $400 for what is probably a $2 capacitor fix.