I have been watching the mythtv mailing list, and a significant change is in the works concerning the way guide data is received. The normal guide data services is canceling service starting 9/1/2007. The mythtv and xmltv guys have started a new service called http://www.schedulesdirect.org/
this service is supposed to take the place of the datadirect service currently in use.
I also noticed that a recent SVN commit added this new guide data into the --fixes branch, which is the current branch used in LinuxMCE (I believe).
This new service is not free, but does require a subscription. unfortunately there does not appear to be many other options for North American users.
Any word on updating mythtv to the most recent --fixes branch so we can utilize this service if we desire.