This armband claims to fit Droid X and smartphones over 4.5"
Tune Belt Sport Armband for HTC EVO 4G, Droid Incredible and more (fits larger smartphones and iPhones in protective cases like iPhone 4 in Bumper case)
Just ordered mine. I will let you know.
From the Amazon page:
Designed to fit HTC EVO 4G, Droid X and other smartphones over 4.5" long
Ok, so I got the armband and overall it is pretty good. It's not spectacular but it's pretty good. The only major complaint that I have is that the slot that the phone goes in at the top doesn't velcro shut or something. Probably ok since the phone goes in fairly firmly. The armband fits over my arm fairly well and I am 6' tall around 200 lbs with 15% body fat. Someone who is much larger than me would need to get the arm band extender as the velcro was reaching close to the end with only about 2" left to extend and it was fairly snug on my arm.
Here is a pic with my Droid X in the armband: