In case you've found Jeff Miccolis' post about the Extendr module for Flickr great, but have been bothered by the fact that the link on the blog was broken and that comments on that article were closed, the good news is that he made his code available on drupal.org too, at:

http://devseed.svn.cvsdude.com/sandbox/drupal-6/extendr/

As explained on the original post and as you should guess given the source code of Views, this needs a patch to Views 2 to allow overriding views_plugin_query_default, or could likely be used on Views 3. The beauty of it is that it's so short when you look at the implementation. Hopefully someone at Devseed will eventually fix the link...