I bought Nokia CP-402 carrying case for cycling use and I am quite happy with it. I could think it is suitable for jogging also, but haven't yet tried that.

It just about is also big enough, when stretched, to fit N900 with Mugen's thicker back cover. The case is in its limits with that though.

The case does not have opening for proximity sensor, so answering phone calls without handsfree is difficult. One can answer though by activating screen with the physical slide switch and then touching Answer/Reject buttons.
There should be a way to answer phone call without touch screen somehow...like double pressing the power button!?