Wikileaks has the documents.
UPDATE (September 19, 2008): The screenshots used by the attacker showed that he used ctunnel as his web proxy, and contained enough information to identify his source IP in ctunnel's logs.
As pointed out by commenter Schtacky, it looks like they've identified the culprit, who used some Google research and Yahoo's password recovery feature to change the password on the account to break in.
This shows the problem with choosing "security questions" for password recovery that have answers which are easily publicly available.
I hope that this kid's actions don't sabotage the corruption case against Palin that may have been supported by evidence in her Yahoo email, evidence that is now tainted by the fact that it was compromised (and subsequently deleted).