I just wanted to add my vote for this feature.
I just received a new Roku 4 as a Christmas gift which I now need to return since it doesn't properly work with my legacy, non DD+ capable, receiver.
Dale -when I managed to find a DD stream the Roku correctly passed it. Unfortunately, that seems to be rare at least with the content providers I use. Amazon seemed to be using DD+ exclusively, VUDU seemed to be a mix with newer stuff being DD+ and older stuff being DD. I don't understand if the fault lies with the content providers or the Roku, but in the end I consistently get 5.1 from all providers with my $70 Sony blu-ray player. While the search capabilities and speed of the ROKU interface is certainly inviting they aren't compelling enough for me to upgrade my AVR.