Some interesting (and a little concerning) case law precedents have recently been set in the USA, where lawyers have successfully sought orders from the judge to have their opponent hand over their login details for facebook and myspace, so all their "private" messages and restricted "friend-only" content can be revealed to the court and used in evidence against them.
Main requirement seems to be that some public content is posted that suggests the person has been lying in their evidence, which then justifies the court making an order to hand over access to private content. So these rulings are limited in scope to some extent, but the judge's comments in one trial give quite an insight into how this sort of online content may be viewed by the courts...
"...Jefferson County Court of Common Pleas Judge John Henry Foradora granted the motion, noting that Facebook and News Corp's MySpace are specifically designed for sharing personal information.
"While it is conceivable that a person could use them as forums to divulge and seek advice on personal and private matters," Judge Foradora wrote, "it would be unrealistic to expect that such disclosures would be considered confidential.""http://www.stuff.co.nz/technology/digital-living/4594843/In-US-courts-Facebook-posts-become-less-private