This i s a well-known phenomenon - people notice improved sound after a guitar warms up. Try this thread on it:http://126.96.36.199/forums/showthread.php?t=188116&highlight=warm
It seems to happen most on acoustics and particularly on archtops, but I don't pretend to understand the theory. It happens on my Martins as well as my Avalons. We probably notice it more on good quality guitars as they are more sensitive.
Banjos are different - I have an old banjo with an original 1920s calf-skin vellum on the drum. It sounds much better when cold - I think it is to do with the temperature changing the tension on the vellum. They also sound better in dry weather!