I have, and we'll probably use it on some level, but it doesn't really apply to unit-style testing, or even what I would classify as functional testing. It's much more about acceptance and regression testing. I find the tests too verbose to write by hand; there's a nice Firefox extension for recording, but you can only record after the application is written.
i also have a cherrypy filter that i wrote that lets me write Selenium tests in a much less verbose syntax (no ugly html table code). i could share it if anyone's interested.
i still use py.test for most of my testing but for acceptance testing of web apps, Selenium has proven itself extremely useful. there's something extremely comforting about seeing that everything works together from end to end all the way to the browser.