I am very excited to say that I had a crazy theory that ended up panning out. I hope it works for you too DDekker.
I thought that maybe since my Windows Media Server didn't work on the PS3 (but could be seen by it), maybe getting the Windows Media Server to work with the PS3 would also get the PS3 Media Server to work. Who the hell would think that I would be right...
This is what fixed it for me - I followed the instructions of McSquishy and Da_Best_12:
What baffles me is why this wasn't an issue for me before until I got the Actiontec router. Never over the past few years have I altered any of the Windows 7 default media server settings (nor have I ever even used Windows Media Server to begin with), and I've reinstalled Windows 7 at least 2 or 3 times in those years. It's as if the Actiontec router somehow amplifies the weaknesses of Windows' default configuration.