My problem turned out to be erroneous network settings on my part. For some reason I had my network adapter's NetBIOS setting turned off. After enabling that (couldn't map the NAS otherwise, as noted in another thread) and refreshing the DNS321's UPnP AV server, now the PS3 (firmware 2.80) access and play all the mp4, avi, and mpg video files currently on the DNS321 (fw 1.03 beta build 12).
Also maybe should mention, there was one corrupt avi file in the tree, and I replaced it with a good copy.