I'm not sure if it's been done before but has there been a contest where participants qualify by posting HELPFUL or INSTRUCTIVE tips re webmaster tasks/responsibilities. These can take the form of technical issues, productivity boosters, or 'self-help' type posts. Also they can't be cut and paste from another site. Authenticity will be verified by running the entries through text verifier --
http://www.webmasterlabor.com/tools/checker
Pro: Subsidizing a positive, business-related thread that is useful and will probably be bookmarked by some webmasters
Con: How do you judge such a contest since the entries are so disparate? It seems extremely subjective and might turn off more webmasters than it helps. How do you limit the scope of the contest?