Member of 18F here. Not sure about USDS, but we get "public trust", which is basically the step below clearance. Basically means "stuff that isn't public yet". http://en.wikipedia.org/wiki/Security_clearance#Public_Trust...
Protip: Contractor or FTE, make sure you specifically ask HR whether or not there will be an additional screening, and whether or not it is a 'public-trust' position and what level that would be following your hiring.
I was pretty miffed when 'they' told me after the fact that my position required an additional background check and that my continued employment would be predicated on the outcome.