I got my WMC8 keys like 2 weeks ago and installed them just fine. I don't use wmc very often, just made sure I got them free with both my w8 installs. It plays my movies just fine though.
Rig: i7 2700k, asrock extreme 4 gen3, 60 gb OCZ Vertex ssd, 128 gb Plextor M3 ssd, samsung spinpoint 1tb hd, 16gb gskill sniper series, EVGA GTX 670 FTW, antec hcg 750w psu
Server: Intel g630, Biostar H61, 4gb gskill ram, antec 400w eco psu