Since its getting close to the most wonderful time of the year, I figured I would toss this one out there as a must have if you exercise with your phone. I use an Armpocket Xtreme i-30 (there are other models). I had to use this one due to the size of my phone (Android Galaxy Nexus). There are 3 armband sizes small/med/large and an assortment of colors to choose from. While this is a somewhat bulky armband, once your moving you hardly notice your wearing it. Other than your phone, there is a small area to hold keys, slots for credit cards/id, or anything else that is about the size of your phone. The back of the armband has memory foam which will fit snug to anyone's arm shape and tbh it works a treat. I know there are other options out there, but since I always have my phone and sometimes jog at the local high school track this suits me perfect. I want to say I've used mine for about 6 months now with 0 issues. I don't think you will be disappointed if you buy anything from http://www.armpocket.com/ If you hunt around they sometimes have free shipping coupon code.
Here is a pic of the inside. My Nexus would fit right up in the top half with the strap plus plenty of room for other goodies behind it.