You could be burning a little bit of oil. Keep a close eye on your oil consumption over the next little while to see if that is potentially your problem. The only other thing that I could think of is perhaps your timing is a little off, so have a look at that. If your not quite sure how then have a look at this page. http://projecthp.ford23.com/tech.html
Keep in mind that if your car is a 91-93 then you will not be able to adjust your timing because of the distributorless ignition. 87-90 is still do-able though.
Perhaps your 02 sensor is malfunctioning slightly and causing your fuel/air mixture to be rich on start-up. That's about all I can give ya. Good luck.
Yeah i would have to agree with valve seals. What happens is after you shut your car off the remaining oil seeps down in and collects in the combustion chamber and when you start it up it burns it. If its not smoking when you drive its because its not leaking enough to where you would notice it. Only when its not running is when it has time to accumulate.