I was looking for a slip case for my 8830. I wanted one like the one that is supplied with the Curve. The Curve case is unfortunately too small for the 8830 because I tried it.
I bought the Gizmo slip slip case for the 8830 but it is a recycled case for the 8700 series. It is not suitable for the 8800 series as the device can slide out easily and be damaged.

to Gizmo for that case - avoid!
I bought a Rex Regina Morgan slip case in Black Lizard. It is very good. It has the sleeper magnet and a cut out for the roller ball. It looks very elegant.
Only two comments. It only comes in black. The cut out for the roller ball should go all the way to the top of the device because removing the device from the sleeve still can move the track ball.
Nevertheless the only game in town for a case of this type. It will also work for the Curve but its a wee bit large.
It's expensive too!
RexRegina Morgan Case >> BlackBerry Pocket Pouches >> BlackBerry Cases >> BBerry)