Update:
Based on some additional troubleshooting, this problem is independent of Theater View. It is actually related to the use of the G-Force visualizer in full-screen mode (I use G-Force all the time in Theater View, so I mistakenly assumed that's what triggers the problem).
As soon as fullscreen display mode is engaged with G-Force active, Girder begins to use substantial CPU time for no apparent reason. It appears to be receiving messages from somewhere (the message count increases), but the Girder logger shows no events from input devices, so the messages it's processing must be either imaginary or generated internally (program-to-program).
Perhaps MC13 sends messages to G-Force that are being mistakenly picked up by Girder somehow?
Very mysterious (and annoying) problem...
Edited on 3/22/2009 @ 1515 EDT:
Further update (although I hate to keep posting to my own thread)...
Additional testing suggests that this is almost certainly a problem specific to MC13 and how it implements full-screen mode. The problem doesn't occur when G-Force is used in stand-alone mode, nor does it happen in MC12.
For now I have been forced to down-grade to MC12 to keep from frying my CPU. I woulkd appreciate any and all suggestions as to how to further troubleshoot the situation.