Same issue here. The DNS-323 shows up in Media Player under Other Libraries, but I try to access anything it says "The remote media library did not allow the connection. Please contact the device manufacturer for more information." I suppose that's a step up from Media Player on my Vista box though, as Vista's Media Player doesn't list the DNS-323 at all...
I do have the UPnP server turned on, but as I don't have an XBox 360 or PS3 yet I can't really verify that it's working at all...