Details of what is currently known about can be found here: http://pastebin.com/f652c44fb.
This could be huge. In my IANAL opinion, Palin exercised very poor judgement using a yahoo address to conduct state business and treaded on very questionable legal grounds. The state can afford mail servers, and I presume Alaska has them. Palin is accountable to the people of Alaska, and she should exclusively use the state's email servers to conduct state business.
I have to assume that the person(s) who hacked the account downloaded the contents and will redistribute them over the Internet. If there is any evidence of Palin's involvement in illegal activity, expect to hear about it soon.
It would not surprise me if she resigned from the campaign and resigned her office to spend more time with her family.
EDIT: Wired's Threat Level is also picking up the story: http://blog.wired.com/27bstroke6/2008/09/group-posts-e-m.html
Wikileaks has the files here: http://wikileaks.org/wiki/Sarah_Palin_Yahoo_email_hack_2008
Ars Technica: http://arstechnica.com/news.ars/post/20080917-palin-e-mail-hack-makes-case-for-sticking-with-gov-e-mail.html
Security Focus: http://www.securityfocus.com/brief/822