Interestingly, on a Titanium G4 running OS 10.4.11, my wireless connection via FiOS router worked fine until last weekend. Suddenly, I cannot obtain an IP address even though I am logged in to the FiOS router, the signal is strong, and the router settings are appropriate; in fact another Mac notebook in the house connects wirelessly just fine.
I have rebooted, reset PRAM, and repaired permissions without success. I am online now through my Airport Extreme wireless (whose signal is weaker, which is why I was using the FiOS wireless instead... on a different channel).