Cold fronts definately slow the bite for most fish. It isn't the front itself in my experience, but the high and breezy sunny skies afterwards that are tough. Just before a front is usually great fishing unless your talking gale conditions.
For trout, cold fronts don't seem to have much effect, but trout are so vicious I don't think it matters. However, the high sunny skies following a storm are terrible times to fish for lake trout. Go figure.