Driving home the other day, I heard a story on NPR about downy mildew on basil. This story has been getting a lot of attention from home gardeners around the country, so I thought I would pass it along in case you start getting calls: http://www.npr.org/templates/story/story.php?storyId=128061773. Here is another article with a number of photos that you might find useful as well : http://vegetablemdonline.ppath.cornell.edu/NewsArticles/BasilDowny.html.
The basil downy mildew seems to be somewhat like the one on cucurbits in that it does well in warm weather, whereas most other downy mildews prefer cooler weather. We did see downy mildew on basil from a home garden in Baton Rouge last September, so we will likely see it later in the season. Remember that basil will also get gray mold (Botrytis) and tomato spotted wilt, which may be mistaken for downy mildew and vice versa.