What gives, Excel?
Well, it turns out not to be Excel's problem at all. Windows (Vista and 7) are to blame in this case. These versions of Windows use the Ctrl+Shift combo to switch keyboard layouts (by default, even if you don't have multiple layouts). Here's how to fix the issue (see this MS KB article):
1. Click Start, and then click Control Panel.
2. Double-click Regional and Language Options.
3. Click Keyboards and Languages, and then click Change keyboards.
4. Click Advanced Key Settings, and select Between input languages.
5. Click change Key Sequence.
6. For Switch Keyboard Layout, select Not Assigned.
7. Click OK to close each dialog box.
Once you've done this, Windows will no longer hijack Ctrl+Shift, which will allow Excel and any other program to accept this key combo for use in keyboard shortcuts.