The Times has a great story about Kevin Smith and the upcoming Clerks II, including this little tidbit about what he's doing with director commentary...
In an ingenious new ploy, he has recorded a commentary for "Clerks II" that will be available for free download on iTunes, encouraging viewers to take their iPods to the theater for a second viewing.
Brilliant. Multiple viewings, reward your best fans, etc., etc. Why hasn't this been done before?