Whilst custom cases are indeed OT, antenna considerations may not be. As mentioned in post #1194, I've replaced the housing of one of my N900s. I still plan to write up a proper summary, although I have a lot going on right now.

A bit of warning - the two strange, detachable, bits of plastic in the back cover (under the battery cover, i.e. hidden from normal view) with the mounting screws going through them are actually the GSM and WiFi/Bluetooth antennae, respectively. That is to say, they have some circuitry inside them. The housing that I purchased had those bits, except they were just plain bits of plastic of exactly the same shape, with the result that the phone did not work unless I used those bits from the original housing.

So I think it's worth emphasizing due care when choosing any aftermarket cases. Sorry if I am stating the obvious, better safe than sorry.