Is this chip a requirement?

Yes, I mentioned that.

As to the actual hardware, again I don't know exactly without going through all the options, but magnetometer tied directly to a bluetooth modem is probably not the best one from a size / cost perspective. You are also looking at modules which are expensive and bulky.

I am suggesting a custom board if there was going to be use beyond R&D.

Anyways, I'm going to wait for the n900 to be in my hands just in case this is some kind of bizarre easter egg (wasn't the radio tuner an easter egg in the n800?) In mid october maybe I take more time to look at it if nobody beats me to it..

Another question I have is if there's an existing library to take raw counts from the digital compass and turn it into a heading. I know there are differences also between true north and magnetic north and I don't know the magnitude of those differences.