The "chin" has nothing to do with it.

I can hold the G1 in the curl of my fingers. Lay the phone across you fingers. Flex your fingers so that the sides of the phone are firmly held by your finger tips and the pad of your palm that is at the base of your fingers. Easy easy (though, the problem with doing this on the N900 wont be its size, it will be the fact that its surface is probably going to be smooth, like other Nokia devices... instead of having a rubberized surface like the G1).

It's secure, and leaves my thumb free to access the entire screen. If I'm holding the phone one handed, that's how I told it. Portrait vs landscape is the same (in terms of how I hold the phone one handed). The only difference, there, is a twist of the wrist.

I only hold "the handle"/chin if I'm using the phone two handed.