Definitely 5.1 and not 2.1, separate surround sounds and yes, the display on the Z-5500 front panel clearly says "Dolby Digital" with the little logo.
It is NOT Dolby Pro Logic II or anything of the sort.
A big thanks to all who reported this capability in the face of so much skepticism. I was a big skeptic myself. I assumed this would never be possible from my past experience and the "copy protection" excuse makers had been using up to now. But I finally tried it from myself and am happy to report my new Sony 55EX500 does indeed pass DD5.1 and DTS from HDMI inputs to optical just fine. I can now use a single optical input on my amp and have removed 4 optical cables from my system and greatly simplified my remote macros.
EDIT: Well I thought I was getting both, but turns out DTS was getting down mixed to stereo. So I only get DD5.1 pass thru. Sorry for the mis-information. I've had to add back one optical cable for my BD player. My other sources are only Dolby.