Solved: LAN, OPT1 bridging / UPnP / DHCP relay questions
Related to my issues here: http://forum.pfsense.org/index.php/topic,7100.0.html, I figured out why my 360 couldn't see my PC.
I looked in the Event Viewer and noticed that the WMPNetworkSvc Service had an error:
A new media server was not initialized because WMCreateDeviceRegistration() encountered error '0xc00d2711'. The Windows Media DRM components on your computer might be corrupted. Verify that protected files play correctly in Windows Media Player, and then restart the WMPNetworkSvc service. For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Of course, the support link is never any help, why should it be in this case.
After scouring the web in search of an answer, I finally said, "Screw Microsoft. Screw DRM." and installed TVersity.
I quickly realized that Microsoft's implementation of the XBox 360 media features sucks and went back to XMBC :D