There was an interesting post published on WPCandy.com by JD Bentley where he discussed three different ways in which WordPress could be used as a project management application. JD describes three different ways to accomplish this functionality. Using a plugin called WP-Project, using the P2 theme, or using a vanilla install of WordPress. However, JD notes that none of these solutions matches what BaseCamp can do either because the solution is too simple or the feature set is too complex.
JD has recently got involved with the development of WP-Project as he believes the plugin has a lot of potential. If you’re interested in helping out with this aspect of WordPress which Ryan Imel says is ripe for innovation, get in touch with him.