Marketing is an interesting science - I love it (and hate it).
These people have truly got something going when it comes to studying and selling to girls (I'm stating a fact - not saying I approve):
http://www.girlgames.com/Templates/index.dwt
http://www.giaheadquarters.com/
Positive girl's charity:
http://www.girlsinc.org/ic/index.php
A book that comes out on Sept. 7th that you'll want:
Born to Buy: The Commercialized Child and the New Consumer Culture, Juliet B. Schor investigates the myriad ways--some of them disturbingly intimate--American children are being marketed to (there's an excerpt from this book in the current issue of Brainchild Magazine).