Tailable cursors ostensibly exist to allow for developing "tail -f"-style functionality atop capped collections. At present, though, if a tailable cursor gets to the end of its results, rather than blocking while waiting for more data the way tail does, it raises a StopIteration exception, which makes it impossible to know when more data is available by any means other than polling. Per http://groups.google.com/group/mongodb-user/browse_thread/thread/d27ee734bb97845e/390c1b78a9f52a4f the necessary server infrastructure to make this possible was meant to have been implemented in #SERVER-510.