I just got an i2c compass module based on the Honeywell HMC6352 sensor.  Sparkfun builds the breakout board -  
    http://www.sparkfun.com/commerce/product_info.php?products_id=7915
 
They seem to be out of stock, but I found one of the boards here -  
    https://www.zagrosrobotics.com/shop/item.asp?itemid=704
 
This particular chip is nice because it runs on 3.3V.  Hookup could not be easier - I just connected VCC to Blackfin S-32 pin 1, GND to pin 2, SCL to pin 14 and SDA to pin 15.  
 
The default device address is 0x42, which is actually sent as 0x21 (i2c can be a bit confusing).  0x21 is the hex for the ASCII '!' character, and the HMC6352 command to read the heading is an ASCII 'A'.  So the command 'ir!A' reads the upper byte of the heading.
 
This works perfectly.  However, I discovered that the 'ir' command doesn't provide a way to read the lower byte of the heading, as it only reads a single byte, so the 'ir' command needs to be revised somewhat to fix this.  Perhaps the easiest fix is to always have the i2c read command return multiple bytes.
 
In any case, it took about 5 minutes to set this up.  Once the 'ir' command is changed to handle a multi-byte return (the SRV-1 firmware can already do this), this little compass will be quite useful.
 
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update - I added an 'iR' command to read two bytes instead of one.  this looks to be a good general solution, but if anyone can think of examples of i2c devices that would to send longer data streams, let me know.

The compass works quite well, and performance indoors is better than I expected.  I'm using it on a blimp where the motors are more than 6-inches from the sensor, and have not noticed any problems yet with magnetic interference.  If the motors were closer, there might be an issue.

A compass won't drift, so it gives you an absolute reference for direction.  Unless you have some other kind of beacon (you could use visual markers with the camera), a compass is very effective for this purpose.

We use a Honeywell HMC6352, which is a 3.3V compass.  The CMPS03 appears to be a Philips compass, and it requires 5V.  I think you will have less difficulty with the HMC6352

Hi,
 
I will be hooking up an HMC6352 to my SRV. I'm planning to attach it to the centre of the bottom of the chassis (this will help ensure the compass stays horizontal). I was wondering whether I should be worried about interference from the motors in this position. Is there another place for attachment that is recommended?
 
Thanks.

Admin is right about interference.
I have HMC6343 (3 axis compas, advanced version of HMC6352). What I can say..
If you have something with magnets inside within range 30cm around compas then compas will show wrong data.
Without any magnetic things it shows good and stable values.
Compas lies when not connected servo motor moved by hand within 30-40cm.
I am using pan-tilt with servos for small robot and I have no enough additional 30cm for compas,  
so compas is wasted money only and it moved to box for next time

If somebody wants to use compas together with OV7725,
I can recommend to use following device. It works with SRV1.
http://www.sparkfun.com/commerce/product_info.php?products_id=8656
 
I remember, admin told, there is some address collision between compas HMC6352 and camera OV7725 .
I have no HMC6352, but for HMC6343 you can make following:
 
wire pins from S32 to compas (4 wires)
1        3.3V               3.3V voltage regulator  
2        GND                ground  
14       I2C SCL            i2c scl    
15       I2C SDA            i2c sda
 
And also change srv.c:
 
void read_compass() {
           unsigned char i2c_data[10];
           short ix, i, head, pitch, roll;
           unsigned char addr;
           
           // HMC6343
           addr = 25;
           i2c_data[0] = 0x50; i2cwrite(addr, (unsigned char *)i2c_data, 1, SCCB_ON); delayUS(10000);
           for(i = 0; i <= 5; i++) i2c_data[i] = 0x00;
           i2cread(addr, (unsigned char *)i2c_data, 6, SCCB_ON);
           head = ((short) (i2c_data[0] * 256 + i2c_data[1])) / 10;
           pitch = ((short) (i2c_data[2] * 256 + i2c_data[3])) / 10;
           roll = ((short) (i2c_data[4] * 256 + i2c_data[5])) / 10;
           printf("##C head=%d pitch=%d roll=%d\n\r", head, pitch, roll);
}
 
Console command for compas data is: $C
That's all.

Hello
 
Quote from admin on 03/16/09 at 10:52pm:

 
What it the procedure of changing compass' I2C address from 0x21 to 0x22? The topic seems to be stopped with no update regarding this.
 
Does current "$C" command use 0x22 address or 0x21? I just connected the compass and the command always gives me the same number (I did not change any addresses).