Here's a good summary of the Cuthbert case in which playing around in the United Kingdom with URLs (e.g. adding "../.." in front of them) landed a web user in prison. The conviction was probably based at least as much on what Cuthbert said he was trying to do by typing this (testing the security to determine whether the site was a phishing site or a genuine one -- a good but illegal intent) as on what he actually typed.
Poorly written statutes like these produce all sorts of bad effects and injustices. What does it mean for a computer access to be "unauthorized"? For a cybercrime convinction to be just, there should at least be notice (analogous to a "no trespassing sign") and cautionary affordance (analogous to a door or a fence that one cannot cross by accident). Stay tuned for more comments on this here in the next week or so.
Ian Grigg and Adam Shostack also have some interesting comments on this case.