Until my phone was stolen last night I had this exact Belkin Ergo:
http://www.belkin.com/IWCatProductPage.process?Product_Id=512055
And I really liked it. The case was low profile enough to fit in the pocket of my jeans or purse, the strap was sturdy and springy (I tied a knot in it to make the loop smaller) and I was definitely using the **** out of it. The back of the phone is contoured with ridges which looks more helpful for shoulder cradling than it really is, but it does reduce slippage.
I recommend this product! I am buying a second one now, and am grateful you helped me refind the link without having to go through all my receipts.
The Interim below looks very nerve-wracking in terms of where the cable is attached.