A previous post indicated about using search to aggregate data. Whilst an acceptable approach (though with some limitations?) another is to go down the route of providing a custom Data Source

Todd Baginski & Andrew Connell have presented and posted information on how to do this - the Content Monster Web Part. don't be put off by the name (those who have seen Todd present can appreciate the left field naming!)

The article has been posted on MSDN and the slide desk from TechEd also