The size of this phone is big and makes it difficult to find a good fitting armband. I have been using an armband I picked up at Walmart. The branding is Phillips, yes the same as the electronics maker, but I doubt Phillips actually makes the armband. I suspect they are just licensing out their name to be put on the armband. This particular armband is more of a stretchy spandex pocket that fits on your bicep. It does not have a clear plastic screen like most, but you can see the screen through the stretched fabric. The screen is also touch responsive through the spandex fabric. Because the armband is stretchy, it barely fits the Galaxy Nexus with a case on. At first I had to wear this armband with the phone up-side-down due to the location of the head phone jack, but I recently upgraded to Bluetooth headphones so I am back to wearing the phone right-side up.
I have also read good things about Tunbelt armbands. Model AB84 or AB85 will be the closest fit, but I don't have any personal experience Tunebelt.
I bought an armband for the Bionic I had for a few months....decided to hang onto it instead of throwing it in with the rest of the Bionic accessories I sold to a buddy of mine.....the Gnex fits in this armband pretty decently, although it's not quite as "snug" of a fit like the Bionic was, my guess being due to the thickness difference between the devices. The other thing is that (while I haven't actually used this in practice yet....haven't started my get-back-in-shape/lose-weight new years resolution yet....lol!) since the headphone jack on the Gnex is on the bottom, I'll have to put the phone in "upside down" probably. But I really don't think that will cause too much of an issue with performance or anything.
I use the Tune belt AB84 on a daily basis, phone fits pretty good. Its not perfect, I think it was meant for a smaller but fatter phone. Reaching the very edges of the screen are the only issue, and its not really a huge problem for me just takes more concentration.