Ok, you're talking to a Network Engineer here, 15+ years experience and certifications out the backside from Cisco.
You've got me intrigued.
Are you doing L3 Switching between the WD and your media source?
How are you doing this? The WD can't discover systems that are on different subnets, and DLNA, as far as I can tell, can't do it either, unless you're doing bridge groups or whatnot.
On some cheap L3 Switches, doing L3 has a performance penalty, but I thought I saw that you're using a Cisco 29xxG switch, so that won't apply. I'd be curious to see your switch's config.