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.
Who did your full audit. I am looking for an auditor myself and it would be nice to grab someone who is now familiar with Bitcoin
When you start looking around for security auditors, you'll find they make you sign agreements that you can't disclose their name. This is because if you are ever hacked, they don't want to tarnish their own brand. Ironic, right? But I assure you, this is industry standard for these types of things.
But if you are looking for a known and trusted auditor, starting with Matasano (
http://www.matasano.com/) is a good start. It is not cheap.
Mike