I'm having the same problem I think, Graham... but I'm not sure how to post a screen shot on the forum.
I have some tracks that are grayed out with a little green dot at the upper left of each track in front of the track name -- a la "(DOT) Solo Trumpet". There are instruments instantiated in these tracks, and there're notes recorded, but they won't play back.
Nothing is muted, and I looked at the Arrange window's "Region" local menu, and nothing in there is ticked off to indicate that something has changed.... but again, this is a piece from my old Mac that's now got these grayed-out tracks since I moved over to a new mac.
Any ideas?
Kirby