I've been hearing wind of this for a few days now, but finally have a date to share. August 1-21. Burger King will be doing a Transformers promotion. So, if you like grabbing extra Happy Meal-style toys (What can I say? McDonalds' version has the collective consciousness better than the generic-sounding "Kid's Meal.") based on the Transformers, make sure to go to Burger King in August.
Here's a link with such information as I myself have.
Update - 6/30/05: Found an actual advertising picture. Unfortunately, they don't look too good. Time will tell.