This is a good point. Some research on whether this device is obtainable is clearly in order.

All it takes is an I2C interface and you're in business. You can find the AK8973 datasheet on the web, e.g.,
http://www.datasheetarchive.com/pdf-.../DSA-37871.pdf
It's pretty clear from this that the interface portion is trivial.

I assume you went with the Honeywell 3-axis magnetometer? I'm fairly confident the N900's tilt sensors can be used to do the tilt compensation, so the much more expensive tilt compensated one shouldn't be necessary.

In my mind, the question of AK8973 vs. HMC5843 isn't a useful one because they are substantially the same. Ideally, we'd be able to tap into an existing i2c bus and be done with it---we just need the IC and some bypass caps. Going through USB (assuming OTG support becomes available) will probably require a microcontroller and will be bulkier and uglier.

By the way, as nice as the SDIO idea sounds, it's probably no better than doing a USB interface, since SDIO<->I2C will still take a microcontroller, and the issue of mechanically supporting the compass module protruding from an SD slot is not one I want to tackle. USB seems better suited to this. (I say protruding with fair certainty having played around with the HMC5843 enough to ascertain that it is too thick to go inside said slot. The AK8973 is somewhat thinner, but is still too thick when you consider that you need to mount it on a PCB.)

I wish I could find an actual picture of the AK8973 because in my imagination it would look something like the chip in this article: Atheros's AR6003 chip bring 802.11n to cellphones

kwantam: The person who builds a working USB interface for the N900 will be a Hacker Hero.

So maybe the first thing to do is build an SDIO->USB host adapter for the N900, and then all sorts of new gadgets are available...

It's a 4mm x 4mm x 0.7mm rectangular prism. It has no leads, just 16 metal pads on the bottom. Google for "16 qfn" and you'll find images of one.

Yes on honeywell, because again it's available in units of 1 and you can get a module at sparkfun for prototyping. I'm also told you can theoretically hand solder it.

While I'm often willing to void warranties that goes a tad over my comfort level.

What's your target price point, and what volumes are you guessing?

Do we know that the N900's MicroSD slot supports SDIO? And if it does, are drivers available? I seem to recall that licensing problems prevented open source projects from making free SDIO implementations (which is why the Zaurus C-series, which supported SDIO in hardware, had hardly any actual SDIO products they could use).

The N900's MicroSD slot placement (inside the device) is also rather sub-optimal for attaching external USB gadgets. Maybe a hardhack could add a new MicroSD slot into the case (would require cutting a hole in the case) and connect it to the original slot with a ribbon cable. That might then allow you to connect various oddly-sized SDIO devices without needing to pop the N900's lid every time. Not sure if it would be worth doing, though, since the risk of permanently marring the case is pretty high.

Go up to 10 people and ask them for directions to their house. I doubt that 3 of them will use the words, "North, South, East or West". Then ask each of them if they have ever used a compasss. 0 of 10 will have used a compass.

I would love to have a compass in the N900, but for most people it isn't necessary. Only people that were Boy Scouts actually care about this.

Ok, flame me now.

I have no intentions of 'flaming' you

However, I personally would love to use a digital compass with the N900 so that I could add 'automatic directionality' (or whatever you want to call it) to the astronomy program(s) I have written. Similar to the Celestron 'SkyScout', I would like the directional orientation of the N900 to be reflected in the portion of the sky being shown to the user.

In my case, it has nothing to do with giving 'walking directions' to anybody

A digital compass is not for the user to know there the North is (well, it also enables that, but it is not its main point), it is for the applications that are running on the device to know it. That is a pretty significant difference, as it enables augmented reality applications.

For instance, have a look at this: http://layar.com/

me too,
i want it badly,,,