My 1997 4.6 does that too. Normally it only happens after I start it cold, move it somewhere different in the drive way (doesn't get to warm up is the point) and shut it off. Then the next morning when I come to start it it puts out a nice puff of blue smoke. My experience would say that it is a valve seal issue. They shrink and let a little oil drip into the chamber when you start it cold. So that built up oil on top of the piston burns off when you first start it. Not really anything to worry about, but you ALWAYS want to take it easy on your engine until it's warmed up. No worries though, unless you really want to take your heads apart and replace the valve seals.
most definately valve stem seals. No big deal really. Tells you that the valve stems are well lubed.. and that isn't a bad thing. When you start the engine, there is high vacuume. this pulls any oil availible into the combustion chamber. It is the intake valve stem seals that you see the smoke from. The exhaust doesn't see vacuume to pull oil in... but they could be leaking too.