It's not cheap, but try this:
http://tuff-as-nuts.com/index.asp?Pa...ROD&ProdID=723
They're great cases and I don't think you'll find one any thinner. I've had one for each of my last phones, including the AT&T Tilt. Haven't gotten around to getting one for my Tilt 2 yet, it actually fits in the Tilt's case. Just not snuggly.
The best thing for me is it comes with a leather belt loop as an option. Far better than belt clips. If it's on your belt the case won't come off. And if you keep the case in your pocket the belt loop is unnoticable.