I'm the creator of BitGo, so I know I am biased.  For what it is worth, we've already done a full external security audit (expensive!) of the software both client and server side.  The operational engineering that has gone into BitGo is also atypical and has been designed from the ground up for bitcoin security.  We'll be doing another audit in the not-too-distant future.  Peer reviews and security reviews are absolutely essential.