I'll start with the obligatory I'm not a lawyer, and lawyers are the only ones who can give proper legal advice :)
That said there's a number of factors to consider here. First up, there are some sites who explicitly allow security testing (a recent list on Dan Kaminskys blog). From what I've seen these sites provide rules of engagement (eg, No DDoS attacks). If you follow those guidelines for a large company like google, I'd be surprised if they took legal action against you. Of course I guess you could still theoretically be breaking a local law against hacking even so..
Outside that list or other very well known companies I'd be somewhat leary of a page which said "it's ok to hack this site". For example on a site where user-generated content is allowed, how do you know that the person who created the page has authority to make that decision?
In any case I'd be surprised to see a site say "no holds barred" attacking is ok. Ultimately even if it's just a Denial of Service almost all sites have some level of vulnerability..