jimr381 posted an awesome solution to this on the techguy forum:
- Comments are in the Review tab.
- Click on the red "PivotTable Tools" tab.
- Click on the options button to the very left.
- Within the "Display" tab uncheck "Show contextual tooltips"
- Click OK.

That solved my problem!
It led to my next question: does anyone know how to make this setting the default in Excel 2007, so that contextual tooltips do not show (and interfere with custom comments) on every pivot table being viewed or created?