Due to a number of threads in recent memory asking about this subject, I feel it necessary to make this post. These screens WILL NOT work:
You cannot use these screens so stop asking.
Game GearThese screens WILL work:
iPod / MP3 player
The New York Stock Exchange building
Game and Watch
The majority of portable DVD players
GP2X (about the one thing it doesn't do)
Any other integrated device I forgot.
Pocket TV with video input
Hip Gear controller
Most stand-alone car rearview camera LCDs
Any other screen that was designed to accept analog video signals
These will work, but need simple supporting hardware.
Game Boy Advance - video input cartridge, but you need to leave the entire game boy intactWhy you cannot use these screens
Game Gear - Needs the TV tuner. Only works with some systems.
Sega Nomad - Accepts analog RGB, most older game consoles output RGB.
Nintendo DS - You need the GBA video input cart. The entire DS must be together.
PSP: The PSP screen can be hacked to accept RGB, but you may need a board to decode composite or s-video to that point.
These screens use digital signals to produce the image. That's why it's pixel-perfect. Video game systems output analog signals, like composite video, that your TV understands. Game system LCDs have the right hardware to decode the video and make it the digital signals the LCD understands. It's cheaper and smaller to make the motherboard talk directly to the screen than it is to use encoding and decoding hardware, but analog signals are more compatable (as you would need for a home console). Since the companies don't expect you to take those things apart and use the screens, they don't see a need for analog video signals. You could build a decoder board, but this is far out of most peoples' capabilities. (Decoder boards are complicated.)
Long story short: Unless your screen came with all the hardware necessary to take composite, component, or S-Video signals, don't bother.
Foxx373 has some good advice about using integrated screens:
Dont try. Its not impossible, just really difficult. Really REALLY difficult. Unless you know a lot about encoders, bit registers, multiplexers, shift registers, ADC, and how to fabricate really NICE looking circuit boards, just forget about it. You are better of just getting a composite LCD screen that was designed to hook up to a console. There are 2.5" LCD screens that are built into controllers, try and find one.
More info at http://www.portablesofdoom.org/screens.php
If there are any more questions, or you think a revision to the list should be made, post it here. Do not send it to me in a PM!!