Has there been any auditing of Tails code by outside groups? If not, will there be?
And are there any security researchers that have any particular focus on Tails?
It seems the vulnerabilities that get by even the better programmers are almost endless.
It also needs to be on an ongoing basis, because you never know when things will change.

All I know about is this page :
https://tails.boum.org/security/audits/
If you are in relation with security groups, it would be great to ask them for an audit!
Cheers
Yes, I agree with that, Tails is mature enough now.
What about writing a message to a couple of security groups to ask them to have a look?
Something like "Tails is now widely used and trust by a large amount of users, we would like to have your opinion", pointing to the design, test and contribute pages.
Would you like to give a try?
Cheers,
Tom