You need to parse the flashvars variable of the <embed> tag that contains the video. These change around, so some experimentation may be required to find the current variable names. Roughly speaking, you'll want to use a libraries like mechanize to grab the HTML of the page and BeautifulSoup to parse the HTML and extract the flashvars field of the <embed> element. Then look around at the variables to figure out which one contains the video URL.
e.g.,
br = mechanize.Browser()
# Browser options
br.set_handle_equiv(True)
br.set_handle_redirect(True)
br.set_handle_referer(True)
br.set_handle_robots(False)
# Follows refresh 0 but not hangs on refresh > 0
br.set_handle_refresh(mechanize._http.HTTPRefreshProcessor(), max_time=1)
# User-Agent (this is cheating, ok?)
br.addheaders = [('User-agent', 'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.1) Gecko/2008071615 Fedora/3.0.1-1.fc9 Firefox/3.0.1')]
br.open('%s?v=%s' % (YOUTUBE_URL, vidId))
soup = BeautifulSoup.BeautifulSoup(br.response().read())
flashVars = urllib2.urlparse.parse_qs(soup.find('embed').get('flashvars'))
# Return the first second video source URL
return flashVars['fmt_stream_map'][0].split('|')[1]
