Looking for a bivy for my nomadic life.

The Jak Pak!

Its made for homeless folks.

http://jakpak.com/home.aspx

For extreme cold wouldn't a water resistant bivy be best to avoid condensation for Goretex type materials (think eVent was rated better)? Then again nomads migrate (kind of implied in the name), so would traveling down South be an option to avoid heavy winter gear and insulation?

I would just wrap myself up in a poncho in town, assuming you would be sleeping in a doorway or under a bridge. The chances of getting ripped off are high and you may find yourself trapped in a bivy— quick exit is a good thing. The poncho can be used as rain gear of course and is even better when sitting. Consider a camouflage one for increased stealth camping options. An umbrella is another good nomad/urban tool. I would go with a good tough CCF pad to survive sleeping on concrete.

I once saw a guy day sleeping in a hammock in a forested park and was impressed with the stealth camping opportunities. As above, a camo tarp might help.

Funny, but as a I write this, I was struck by how much safer I would feel in the woods. The idea of sleeping outside in a city feels so threatening.