The handling of the aircraft would be different due to the stiffer wing, and the ride would probably not be very comfortable in turbulence.
A certain amount of wing-flex is desirable, and carbon-fibre wingspars are very stiff. For example, DA42 uses carbon fibre widely in its construction, but the wingspars are GRP because CF would be too stiff. Also, if ever you get to fly a CAP10B (wooden spar) and a CAP10C (carbon spar) you will feel the difference.
Apart from that, there should be no reason why you could not use a carbon spar in an aluminium airframe. There are lots of ways to mechanically fasten carbon-fibre to metal components though, as noob points out, probably expensive and time-consuming to mass produce.
Last edited by Trim Stab; 28th Aug 2012 at 06:00.