A month ago some hackers already managed to use third party codecs , by using FFDShow.

Also DivX did find a workaround, now with enabling MKV file support in the Media Foundation framework.

The advantages of enabling MKV file support in the Media Foundation framework include the ability to use hardware acceleration for H.264 video decoding via the system H.264 decoder (where supported), as well as adding support for serving both DivX & DivX Plus HD video through Windows Media Center Extenders & the Windows Media Network Sharing Service for UPNP devices. However, be aware that transcoding creates an additional CPU load on the server, some loss of quality & potentially reduces picture resolution.

The DivX Tech Preview will demux MKV files and allow the video and audio streams to be decoded with native Windows 7 codecs.

Look here for more info & downloads (registration required).