Tether is supposedly tied directly to USD. Their website says as much and claims transparency through audits. There hasn't been a single audit that has been independently verified.
Right now there have been nearly [800m tethers minted](http://omnichest.info/lookupsp.aspx?sp=31). They are minting them at an alarming rate as seen [here](http://omnichest.info/lookupadd.aspx?address=3MbYQMMmSkC3AgW...).
This is of course circumstantial. I just thought I'd share as I'm stepping back from the markets as a result. @bitfinexed on Twitter seems to be the loudest voice on this topic.