How can I participate in a mining pool that is mining Litecoins? To date I've only participated in mining GPU-optimised crypto currencies. I wondered what happens when I pointed my GPU miner to a Litecoin mining pool worker, Litecoin being designed to be optimised for CPU mining.

I tried do this for just three shares on coinotron.com and according to guiminer (running Phoenix) two of them were rejected as stale and one was accepted. However, my worker on coinotron.com registered no shares submitted at all (which is probably correct, since it's likely a GPU miner won't work at all for Litecoin).

If I can't use Phoenix, then what do I use to participate in a Litecoin mining pool?
Litecoin doesn't use the same algorithm as Bitcoin. Bitcoin hashes are SHA-256 based.
Litecoin hashes are Scrypt based. You must use a miner capable of understanding scrypt.  Early in Litecoin history there were no GPU capable miners as the performance of GPU relative to CPU was very poor.  In time GPU performance increased and today most mining software supports both SHA-256 and Scrypt.  Part of the reason that GPU mining is possible is that the Scrypt parameters used in Litecoin (and other clones) have been selected to reduce the "memory hardness" of the algorithm.  The default parameters for scrypt are n=2^20, r=8, p=1.  The Scrypt used by Litecoin uses the parameters n=2^10, r=1, p=1 making it roughly 100x less memory hard and allowing GPUs to out perform CPUs while they are unable at n=2^20, r=8, p=1.