Doug,
Having a client who has products based on the PPC 405 and 750, I am definitely in expanding into systems which run on these processors. Unfortunately, their systems are very expensive, and so I am limited to working remotely on systems in their lab. However, I have been looking for an inexpensive implementation such as the Walnut EVB, with no success. Your mention of being able to get a board makes me want to know more.
I am not entirely sure what you are looking for in a system, but a while back I worked on a couple different borads with 405 processors. Both ran NetBSD wonderfully, although the CerfCube required a few kernel tweaks to accomodate its 405EP processor (as opposed to the GP). Both are good systems to prototype stuff on - and very reasonably priced.
CSB472 (IBM405GP), by Cogent http://www.cogcomp.com/csb_csb472.htmCerfCube 405EP (IBM405EP), by Intrinsyc http://www.intrinsyc.com/products/mob_ref_sys/cerfcube_405_ep/index.asp
Best regards, Jared Momose