Steve,
For doing RDM Development, probably one of the more useful things is going to be the ENTTEC USB/DMX stuff. There's some discussion about it in the "RDM Marketplace" forums here:
http://www.rdmprotocol.org/forums/forumdisplay.php?f=16
Other good RDM Controller products I know of are the Wybron InfoTrace system, Artistic Licence nodes and Jumpstart, as well as the RAD from Doug Fleenor.
The ENTTEC interface and possibly the Infotrace are the only ones that I'm sure have been updated to the final version of RDM (including START code). I know Wayne at Artistic Licence was in the process of updating everything to the final version of RDM, but am not sure where he's at on that.