Englishal,
A system was reviewed recently in one of the French GA magazines (Aviasport or Aviation et Pilote.... can't remember which). It consisted of a small "black box" and a Compaq PDA. The box had to be attached to the floor as close as possible to the centre of the aircraft. It was very small and light, and contained gyros of some kind.
The PDA then became a backup attitude indicator.
From what I recall, the results were a bit mixed. Most of the time it worked very well. Once or twice it decided it was banking when it wasn't....
The manufacturer reckoned the bugs would be fixed easily (of course!), and they plan to sell the system for the equivalent of a few hundred pounds.