Thanks for the replies, folks! To answer the questions and suggestions:

rharv: PT 2011 (1) (I assume the 1 means build number...?)
I skipped PT 2010 (because for some reason didn't hear about it), but have used PT since version 7 or 8 or so.

While I'm at it, I should mention that I'm using Windows 7, 64 bit version.

jford/redguitars:
I checked my Sticky Keys setting and as far as I could tell, it was off. I went so far as to disable Windows' ability to automatically turn it on (with the 5 Shift presses in a row).

I tested it again, and still have the same problem, so that doesn't seem to be the solution.

redguitars (or anyone else who may know):
I'm willing to call support, but will it help? What can they do at this point?


What seems especially weird is this: When I go to the Bars window itself, the letter 'p' does NOT toggle Part Markers but plays my song (though why 'p' is used for play, when the space bar already toggles play/stop, is a design decision that will remain a mystery to me unto my death).