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FPS games are so easy primarily thanks to Id, which has consistently posted the source code to their previous generation engine the Christmas after the current engine comes out. Carmack has already posted on his blog that the source for Doom3 has gone to legal for final vetting for release this xmas, so look forward to a big jump in linux FPS games next spring.
Look at all the awesomeness that came from the Quake3 code... ioquake3 maintains and extends the code, which in turn is the base for games like Urban Terror, World of Padman, Tremulous, and more! I can't wait to see what comes from the Doom3 code!