[Huckleberry]

Well I've noticed that guitars I've bought new seem to lose a bit of 'stiffness' fairly quickly (like over a few days). But I also noticed this later on, particularly with my Martin HD-28 - whenever I hadn't played it for a while it would feel and sound stiff for an hour or so, then relax.

I'm sure guitars do 'open up', but I'd expect it to be a pretty subtle change over a long period - difficult to quantify, and there are so many other things that affect the tone more radically (strings, temperature, humidity, acoustics of the room you're playing in) that I would find it really difficult to isolate one aspect of the sound and say "that's due to the guitar opening up".

In terms of fallibility, changes and situation basis of memory over time - absolutely. Read any text on Neurolinguistic Programming, and that'll give you some methods to do it proactively.