We respect SE's attempt to get into the jogger's market with the armband for their W710i. The only problem is the phone is huge, relatively, and putting it on your arm doesn't make it any thinner.
Compare this shot with a shot of a Nano armband. Much smaller, much sexier (thanks to the model), and it doesn't look like a tumor's made its way up your arm.
Pictures of Sony Ericsson W710i fully armed [Mobile Mentalism]