Natalie,
"Nana" should be plenty winter hardy, in the ground, in zone 8 - I've even seen it in-ground at the St. Louis Zoo, so it may be able to handle zone5/6 winter conditions.
The fruits are edible, though I don't know how tasty they may be. This one's been selected for its dwarf habit and blossoms, moreso than for fruit quality.

Thank you Lucky_P. I will consider "Nana". Pomegranates are one of my favorite fruits! Always look forward for them to appear on supermarket shelves.

Natalie,
The dwarf you saw is most likely 'Nana'. Nice, if you're looking for a pretty little flowering shrub, but if you want pomegranates for eating, I'd advise getting a selection made for fruit production. They're winter hardy in zone 8 and parts of zone 7, in a protected site. There are even some Russian selections that may be zone 6 hardy.
'Wonderful' is the old standard variety, but there are others that are better/more productive.