Looks like they still are! RDU is planning to build a “Consolidated Rental Car Facility” (CONRAC), among other things. See future building L-5 on this finalized sheet drawing that the FAA approved (warning: large PDF).
While they were deciding on that option, though, it looks like RDU eliminated a design that included an in-airport people mover (see Alternative 8). It looks like they got rid of it since that sort of system doesn’t make much sense if it’s only around the airport…
Of course, that doesn’t mean you can’t have a rail link that also blends into both existing and proposed buildings. But you still need additional approval from the FAA if you want the regional (not just commuter) rail system to go through the airport, though, on top of the commercial/safety arguments and environmental studies you gotta have no matter what.
On a more general note, though, having a regional rail line that connects to RDU (by bus like @CanesFan said, or by ecoPRT) could make the case and demand for expanding to Fuquay and elsewhere too.