This looks like it could be a very entertaining site especially for 8-12 year olds. Even adults, like me, who enjoy reading an occasional children's book are intrigued! It combines reading books with online searches and will begin in the fall. Here is a link to their official website (which is still under construction) that provides an informative clip about the game.

Thanks Fleur! Looks like fun! 

I did a quick search for some more details. The release date is 9-9-08. It is nice to see a major company like Scholastic doing something like this, but as a contest it will unfortunately be expensive to play for relatively small prizes.

PDF with details for publishers:
http://www.scholastic.com/internationalrights/pdfs/39cluesFAQ.pdf

Barnes and Noble pre-order page for the first book:
http://search.barnesandnoble.com/The-Maze-of-Bones/Rick-Riordan/e/9780545060394

Note that while there will be $100,000 in prizes, the grand prize is only $10,000, and you will likely have to buy many trading cards to get all 355.