A while ago, I wrote about how t-mobile silently traps SMTP traffic. Boing Boing has an article about some one who as done a presentation on how ssh could be tunnled through dns.
This seems like this could be a way to get free internet access from an internet cafe. But, I also think that this would be pretty easy to block. All a dns server would have to do is to not forward dns queries that are not in a known format.