I have a 2010 21" iMac on OS 10.5
I had a recent scare, when my iMac could not get past the startup screen, and was stuck with the turning wheel, and I could not get onto the log in page.
Initially, the iMac was also stuck when I tried to log into "Safe Mode", and pressing the option key (to choose which startup disk to use) also did nothing. I tried using the installation DVD, which also did nothing. I could initially hear the whirring of the hard disk, so I assume it wasn't a HD failure.
I then pulled out the power cable for a few minutes, and then tried the option key again. This time, I could get to the choosing HD screen, and I have previously installed Tech Tools Pro, and used the emergency drive to at least boot. I then used Disk Utility to check the HD and it was fine, so it seemed like the start up process was corrupted.
If you insert the DVD and pressed C, nothing happened, but I got to the emergency drive, then inserted the installation DVD, then I could choose that, and using that I then could access the Time Machine on a separate HD, and restore it to a previous date / time. That took almost 1 hour to copy over the restoration. I then booted first into "Safe Mode" and copied over all the recent files I had changed.
Then with fingers crossed, restarted and viola, happiness!
Hope this helps anyone with a similar problem. I think the startup directory or file got corrupted.