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I had no problem getting to my Yahoo! fantasy baseball pages thru the iPhone Safari browser. Can set lineups / add / drop just like on the computer. I don't think GameCenter will work since it is Flash, but SportsTap works for checking scores / stats.
 
Yahoo! FantasySports iPhone App

Check out my newest app called Y! Fan at www.yfanapp.com

It just got added to the app store and is now available for download. It provides an iPhone specific interface to manage your Yahoo! FantasySports teams. You can set your roster, view your standings, track live scores via boxscores or stat tracker (if you have it), and more.

Future versions will add support for transactions, message board access, among other things.

Take a look. I know it will accomplish exactly what you (and hopefully thousands of others) are looking for.
 
I'm very surprised that an application for Fantasy Football Leagues haven't been made yet(not just one for yahoo).
 
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