You pretty much got it covered, Don. Is the smoke bluish, or bright white? If the "smoke" is white, I'd also be concerned about the possibility of moisture getting into the engine in some way. Does it miss or run rough while it's warming up, or put white smoke out the tailpipe? I've seen a leaky head gasket that will seep just a little moisture into a combustion chamber or two and cause a miss and lots of white "smoke" (steam, actually) will be evident right after startup until the moisture is purged out of the combustion chambers. In the early stages of a head gasket leak, there won't be any overheating, just steam after a cold start.
Jerry
