Thanks - let me know when and if you have something test-worthy, and I'll be happy to try it here on both my 10.6.8 machines.
My suspicion is that it's something to do with the way the Mac OS handles Start-up applications in its System Preferences > Accounts > Login Items. In MFC, checking the "Autostart minimized...," box in the Preferences places it on the Mac's list, and I wonder if the Mac is not tricked into starting (or calling) it twice, which might account for the open window. I've tried checking the "Hide" box in the System Preferences, but this has no effect.
MacBook Pro 3.1, 2.4 GHz. Core2Duo
Mac OS 10.6.8 - MFC version 1.2.1
Windows 7 SP1 32 bit Pro via Bootcamp 4.0.4033
Mac Mini 6.2, 2.3 GHz. i7
Mac OS 10.8.5
Windows 7 SP1 64 bit Pro via Bootcamp 5.1.5621