We mentioned the Niner S.I.R. frameset a little while ago on this site but it's worth pointing out again, now that we have a gorgeous Kermit Green model (size large) in stock.
The S.I.R. is made to be run with whatever gear setup you choose. It has an eccentric bottom bracket to pick up chain slack if you run at is a single speed and it also has a replaceable rear dropout with an extra dropout provided, one for a derailleur and the other without the derailleur hanger for a nice and clean single speed look.
The Reynolds 853 steel tubing feels great, and the colour is awesome. As it turns out it is easy being green.