I like the sound of this and all the best in achieving, I would like to suggest to give a grip on the phone, i.e. few positions suggestions
1) Next to Infra-Red Port for holding the phone for camera or to hold the phone with single hand, well I am righty usually I hold phone with right hand.
2) at the bothom of the phone, for supporting the adove hold
3) on the backlid,
3.a) it would look stylist
3.b) helps in opening the slide with less effort.
So, what do you think?

BTW its high time I changed something with N900, it will be a great feeling holding a N900 in a metal body.