During the SpaceX launch of the CRS-10, at T+3:12, there is a circular grey object that passes through the frame. What is this?
There are two other covers (which are actually called fairings) on the Dragon spacecraft during launch that protect the solar arrays, but these are not circular and they are only released after orbital insertion right before the arrays deploy.
The cone is being released at the altitude of ~114 km which is very similar to where "normal" payload fairings are released on other launches (at that altitude the atmospheric drag does not posses danger to the structures anymore). Thanks @uhoh for this part
Credit SpaceX webcast
The nose cone will be a bit different for the Dragon v2 where they designed it to open and close on hinges, so it will not be thrown away but will be brought back to Earth instead.