You could try opening your iMac so you can turn it on and see if the diagnostic leds light up.
http://support.apple.com/kb/HT2831 in particular: Locate the large white arrow in the middle of the computer.
Above this arrow, you'll see four LEDs: LED 1 indicates that trickle voltage from the power supply has been detected. This LED will be ON when the computer is turned off and your power supply is working correctly. UPDATE: If the 1st light is on that indicates that the power is flowing to the computer. So it could very well be a logic board failure. At this point i'm not sure trying to repair a logic board failure is worth it.
Sorry. Make sure you pull the HD though if you want to recover your data.
don't underestimate the power of cleaning.
its a bit complex but http://muzso.hu/2008/08/17/how-to-clean-the-lens-of-a-slot-loading-optical-drive-a-macbook-pros-superdrive has a good walkthrough suplemented by an ifixit guide (ofcourse) on how to clean the lens in the superdrive.
Actually, if you're anywhere near NYC (and even if you're not) give the guys at tekserve a call, it might be easier in person, but if you get a sympathetic tech or manager he may be willing to sell you the OEM part if they have it.
Is the screen actually in black and white or could it be that its in reverse black on white contrast mode http://grab.by/Zgb if it's the latter you can find the accessibility settings in your iPhone and turn it off.
Use it as a kitchen tool for quick look ups of recipes and such
http://code.google.com/p/xbmc-n770-remote/ use its a remote control for a boxee install on a macmini hooked up to your tv or an apple tv running boxee. use it with a full sized bluetooth keyboard or the apple aluminum keyboard http://cs.gmu.edu/~sean/stuff/n800/keybo... as a dedicated writing and email station. This is how i use mine, also use it to have IM running in the living room when i'm watching something on TV.
http://blog.macsales.com/2751-proprietary-cable-can-put-the-brakes-on-upgrading-late-09-imacs this pretty much covers it. So no real solution other than shorting the 2 pins or waiting for a work around. hope this helps
This may or may not be a factor but go to system preference, network, then click on advanced. in the first tab, under airport, check out how many preferred networks you have listed, get rid of any that aren't networks you use regularly and finally if you're left with more than one network drag your home network to the top of the list. hope that helps
I'd check to make sure it's not the LCD by booting the iMac connected to an external display. If you get nothing on the external then you can rule out the LCD and see if its a logic board or video ram issue. UPDATE: If you're not getting video on an external display this starts pointing fingers at the logic board, or possible fans and sensors. My 24" iMac crashed about 4 months after purchase and then would hang on boot, turned out to be the fans and temp sensors.
If you have applecare take it in. If you have another mac, try booting your imac while holding T to boot in target disk mode. and connect via firewire to another mac, if it works you can backup your files before sending it to apple,
apple doesn't really guarantee your data if you take it in under warranty.