This was so easy in SharePoint Designer 2007 - you just used the "Connect to another library..." link and you could quickly create a DVWP on your page that used any list/library in your site collection.
Is there a way to easily do this in SharePoint Designer 2010? Marc Anderson has a post about messing a bit with the code to reproduce but that's not quite the Easy Button I'm hoping for. Neither is using Web Services. Both are fine enough as approaches but they're a both a little too fragile to recommend to some of our power users.