WMI Troubleshooting tips
More from the Configuration Manager team blog, which you really should subscribe to their RSS feed, or make a habit of frequenting their website for some really good ConfigMgr info.
Brian Huneycutt has posted a super cool run down on solving WMI issues over on the Configuration Manager team blog
A good summary of the steps to get WMI back on its feet. And a word of warning about just deleting the WMI repository (which we've all done I'm sure!)