iArmbands Sport Armband for Droid X, HTC ThunderBolt, Samsung Galaxy S & more (fits larger smartphones and some phones in protective cases)
I bought this to use with my Droid X... it just barely fits, and you have to put it in upside down.  They screen is somewhat unresponsive.  However, considering there aren't too many options for large phones like the Droid X, I'm satisfied with this armband.