The mentor signup page for Google Summer of Code 2011 as of this writing still says “We have temporarily disabled the creation of new requests and invites in preparation of the launch of the new UI for Melange later this week.”, as it has said since the 20th.
So, if you’re wanting to mentor, keep an eye on it. I’ll send mentor requests to any of the names on my list of people that have already expressed interest, if I get to a working version of the page before you do…