i got several Tune Belt ("for larger smartphones" variety) armbands for my Droid X, and i was pretty sure the GNex wouldn't fit in them because it is taller, but i'm glad to report i was wrong! the GNex fits just as well as my Droid X, i guess because it's thinner. i recommend these. they don't really have any frills, but they fit and work great:
http://www.tunebelt.com/ab83
Note: i do not work for that company and i don't have the extended battery yet, but i think it will still fit.
http://www.tunebelt.com/ab83
Note: i do not work for that company and i don't have the extended battery yet, but i think it will still fit.