I signed up for a trial membership to CooksIllustrated.com, and get a newsletter. The most recent had this tidbit, which I appreciated since I've always wondered about it.
Are Thai and Sweet Basil Interchangeable?
When developing Thai-style recipes, we've run tests to see if Thai basil can be replaced with the much more available sweet (Italian) basil. The answer in all cases is a resounding no; these herbs have very different flavors. Thai basil, which can be identified by its purple stem, has a stronger flavor and aroma--we detected hints of licorice, cinnamon, and mint. If you can't find Thai basil in your supermarket, we recommend substituting mint, not sweet basil.