How do I diagnose a defective switch in a 4-way setup?
I agree with gregmac that is probably a wiring problem. If it isn't a wiring problem it's probably the dimmer and the dimmer itself may even be the wrong type. You might think about getting another three way switch and replacing the dimmer for test purposes only. If it works with the 3 way switch you know it's the dimmer. If it doesn't work then I would bet the dimmer was miss-wired when it was installed and I would start to see if the carriers and the line/load conductors are on the right terminals of the switch. If that isn't the problem then you probably need on site help to troubleshoot the problem.

I am assuming that most of us don't want to answer this question is because it takes quite a bit of work to try and explain how to troubleshoot the circuitry. Not that it's hard but as my instructors told me "it's easy once you learn how to do it."

Hope this helps.