In order to solve my problem, I renamed the VSTPlugins folder in my environment, in my case:
[System Driver] \ Program Files \ Steinberg \ Cubase 5 \ VSTPlugins
[System Driver] \ Program Files \ Steinberg \ Cubase 5 \ VSTPlugins2
Then I re-loaded my project and voila, it loaded! Now, it loaded, but there were errors, in my case:
The plug-in "Guitar Rig 3" could not be found for Insert 1 Channel "Guitar"!
I removed all references to Guitar Rig, saved the project, and then closed it. Then I SHUT DOWN CUBASE.
Then I changed my VSTPlugins folder to it's original name, and re-loaded Cubase, and then the offending project. Bingo, worked like a charm! The project opened, but I have to re-add my Guitar Rig insert and choose the correct settings. Fortunately, I have presets mapped for most of my songs, but in the case where you don't, you might have some frustration ahead of you.
I hope this saves someone the time I spent messing around with this. I'm glad I didn't re-install Cubase all over again.
I also noticed this same behavior with Battery 3. Those are the only plugins I installed outside of what comes with Cubase 5.