Even without looking at the SONY site I think most people would assume that if you misuse PS3 you are just asking for trouble. I did a search in Google and it turned up this page:
http://www.us.playstation.com/PS3/About which indicates to me that PS3 is indeed a registered trademark. And the site also has a forums section.
I am not a lawyer or an expert, but I would not worry too much about using the domain if you take all possible steps to make sure that no one could be confused about being on a SONY site and include a disclaimer that states who owns the trademark(s).
If you have a good forum then it should only be a benefit to SONY. Of course you should consider removing any talk about things that violate copyright laws and links to warez sites and the like. Don't give SONY ANY reason to not like your site.
It wouldn't hurt to contact SONY and inform them of your plan to create the forum and that you would like to have their permission and know of any rules and guidelines they suggest you follow. They may not know how to deal with this and you may not hear back, or you may get back an rejection but at least you will know and have documentation that you were trying to do the right thing.
(If anyone thinks I have given bad advice on this, please send me a PM with any mistakes and I will revise my post asap. )