An interesting topic came up on the Yahoo! DSTAR_DIGITAL group. One of the challenges to digital over FM voice is multipath (one of the potential causes of R2D2). One method to combat it is found in the IC-2820H mobile with its diversity receive. Another possible combatant might be to put circular polarization at the repeater site.
Reference: http://www.qsl.net/n9zia/cir_pol_rpt.html
There is an article on circular polarization antennas in August 2007 QST. It might be interesting to do some research in this area. Bases could use horizontally polarized antennas to cut down interference from man made sources and be 20 db down from adjacent vertically polarized FM repeaters and mobiles. Using slot antenna designs horizontal might also be practical on 23-cm mobiles.