Has anyone done this yet?
Without them publishing their cold storage addresses this would really not be possible (although I believe that one of the very "high up" mods has conducted audits on many of the exchanges, coinbase included - I believe that circle is too new). Publishing cold storage addresses would also potentially break anonymity for their customers so they would likely not want to do this.
I would say that coinbase is generally trustworthy to hold medium amounts of bitcoin for at least medium amounts of time as they have received VC and the VC firms likely have conducted some level of due dilligance on them
