Okay, I can now completely confirm that my problem's resolved. I just updated to the latest weekly moodle snapshot (as of this moment, of course). Since I've changed my theme file/directory names to all lower case characters, all is well (I had to update a couple of theme files I'd changed too as I had added two lines that referenced the theme's directory name.
Something kind of neat... I tested this a few different ways by way of virtual machine. So I had snapshots and would restore to a pre- "fixed" state to try a few different approaches.
Something that's kind of neat is I'd updated all the theme file names and directories accordingly, but left my frontpage.php and general.php with a reference to my style sheet at [moodleroot]/theme/Columbia/style.css -- the file was "really" located under [moodleroot]/theme/columbia/style.css but I created a sym link (which my web server rules allow) allowing the "C"olumbia version to still be accessible. Suffice to say, this case still caused the above error message. Wacky.
Anyway, moving on...
Thanks for the help everyone! Anyone stumbling on this, should probably take a look at this page.