No you probably can't.
There will be two different sizes, an 8GB and 16GB version. According to the product listing, the 8GB size will retail for $39.99, while the 16GB will retail for $69.99. The USB drives will ship on April 6, according to the listings.
A quick search on Newegg reveals you can grab a SanDisk 16GB stick for about $40. So why the higher cost? The GameStop listing says that Xbox 360 sticks offer "hassle-free 'plug and play' installation," meaning the user won't have to configure his or her own 3rd party drive upon inserting it into the console. But according to Microsoft, having to configure your own drive doesn't sound too difficult.
These retail listings have not been confirmed, so there's a chance these prices may change. We asked Microsoft for comment and will update when we hear back.