I definitely agree with wsc.
check out the vectornav vn-100. its totally awesome as far as embedded AHRS systems. The filtering algorithms are difficult and time consuming to code though. I might consider the $500 price tag if it were open source. With the new 3-axis 4x4x.9mm gyro out it seems like the footprint could be reduced further.
the hardware design of a small format board like this would be simple but the filtering to get the quaternions or a heading matrix out are quite involved and require a lot of tweaking.
an open source product tackling this problem was successful it would be really neat.