I think it's pretty sad that the latest and greatest from Microsoft doesn't support a common format such as M4A in WMP11.  Sure you'll find forums galore with people telling you to try this or try that at your own risk.  The last time I installed a free codec, I wasn't able to hear audio when streaming video from various sites.   Now that's all working and I refuse to go through that again.  If I install anything for WMP it best be coming from Microsoft.

Of course, while Microsoft and Apple duke it out, I can just install a good third party player that will do it, like WinAMP, without messing with WMP11, and just associate the m4a's with winamp.  That won't help playlists, but at least I can play individual files that I have...