Prism is as precisely as "voluntary" as the cops showing up with a search warrant at 5am and asking you nicely to open up "voluntarily", because, well, they have a nice 5.11 MiniRam Breaching Tool 50091 and your door looks very pretty and it would really be a shame to have to knock it off its hinges. Yes, that would be a real mess.

See what I wrote on this last summer: http://news.cnet.com/8301-13578_3-57588337-38/