Wasn't sure if the lower models did 5.1, but it does show up on my XD as an option.
Yes, as mentioned, the source material is the real culprit there. Only a small portion of the streaming content out there is currently 5.1. Netflix streams up to 640K also (DD+), so we aren't talking lossless audio either. No TrueHD, dtsHD-MA or uncompressed PCM like you would get on Blu-ray.
I am not aware of too many devices that are actually 5.1 enabled with Neftlix yet either. The PS3 and obviously a few of the Rokus also, but not sure how many others are. I think there are only like 3 or 4 devices that currently are, but the limiting factor is still the source material.