The Xbox is always going to output Dolby 5.1 if you have it set to output that, whether the source is DD 5.1 or not. (The Settings->System->Console Settings->Audio->Digital Output settings are "Digital Stereo", "Dolby Digital 5.1" and "Dolby Digital with WMA Pro"--I assume that the final setting is for the tiny number of AVRs which can decode WMA Pro, MS' digital surround sound encoding).
HBO Go announced that DD+ 5.1 was coming at CES in January, but I don't think it's available yet.
EDIT: It looks as though DD+ may already be available, but only on some Samsung televisions. AVS Forum member Ranger One (Gabe Gagliano) wrote the following in this
28 February article in his TechoftheHub.com blog:
...the Xbox client will support HD (720p) and stereo sound. Dolby Digtial Plus for surround sound will be added in a later release. For the time being, select Samsung Smart TVs are the only way to get Dolby Digital Plus with HBO GO.