I have a silicon armband case for an ipod video .. I had to cut the front a little bit to allow access to the trackball, but the keyboard is hidden. I also had to stab a hole in the side so that i can plug my head phones in also .. It's jimmy rigged, but it works.
You might be able to get one of those seido or i think fortte rubber cases (i think those are the companies that make them) and cut 2 slits in the back of equal length, and run a velcro band through it to strap to your arm. That way you have full access to your keyboard and headset jack.