Hi I was wondering if the original poster was able to comment on whether this problem is still occurring. I've been plagued with this as well.
I have a 3.06ghz Imac 24" (April release). I started noticing this problem recently. At first the question mark would appear and holding down the option key would still let me choose a boot partition. Eventually a week later the question mark screen was permanent at boot up. Holding the option key down did nothing. Booting up with the MAC OSX CD and running Disk utilities, my hard drive wasn't even visible. I turned the machine off for a few hrs and tried again like the original poster and it then worked. After that I backed up all my photos and made sure to select "startup disk" under system preferences and force it to boot up to my MAC OSX hard drive. Problem seemed to go away. Until today. It happened again after I needed to reset my machine. Option key did not work, waited 30 minutes and was able to boot in with option key held down later. All of this is puzzling to me. When I can boot in, I ran disk utilities on the drive and it says it is perfectly fine with no errors. The only thing I can think of is the recent NTFS-3G driver I installed lately which allows my machine to write to an external NTFS partition hard drive. Perhaps this is confusing my machine on bootup? In any event I've uninstalled it but still got this problem today.
My machine is just over 1 yr old. 500 gb hdd. So I am out of luck getting this fixed under warranty =(
I see a ton of posts about this problem on the internet, but no one really says how they've resolved. Is it a hardware problem? Software? Driver? The problem happens intermittently and I can't seem to pinpoint the problem.
I hope its not a failing hard drive. Anyone know how much Apple charges to replace a hdd out of warranty? I've seen some DIY guides on how to do this myself.
I have this same problem. I've got a 24" iMac as well (2.8GHz, 500GB)
The sad part, I have 280GB of stuff on it, and I have no backup! I just got rid of my old MacBook and got a new one (I normally keep both computers synced together), so then if something happens to one, I have everything on the other and can just transfer, well...of course, this happens when I didn't get a chance to back up the iMac to the new MacBook.
The Apple store has now had my iMac for 10 days so far (glad I got AppleCare!)

Called again today, they tried replacing the logic board, the optical drive cable, and still nothing) Said they ordered a video card.
WTF? A video card would not cause that. She said in the first order, they ordered a logic board, the cable, and a new HD just in case. Well I asked if they tried replacing the HD to see if it was a corrupt hard drive. She said "oh no, that wouldn't cause that" Well.. I'm hoping that isn't the issue, cause then I'll have to go through the hassle of recovering files off a corrupt HD (possible but a pain, good thing my brother knows how to do all those shenanigans). Either way - I don't know what is going on but I want my computer back. I was told it would be done yesterday, now they're saying no earlier than Friday or Saturday the soonest.
