This is where I feel you might be a little bit dogmatic.

You don't need the source code to audit software. Software that are heavily used are audited.

Microsoft software is probably more scrutinized that any other open source one (I'm not implying it's more secure, just that it's more analyzed).

Security is a question of trust, not a question of source code. Even if you do the audit yourself, it's a question of trust: it means you trust your own abilities to evaluate the security.

I'll go a little bit further.

Did you check that the computer you bought isn't rigged? Maybe someone can remotely control your webcam or eavesdrop your keyboard.

Did you check that the operating system you have hasn't been compromised? Maybe someone intercepted your download and patched it on the fly to insert a backdoor.

Is your home physically secure? Maybe someone is copying your hard disk every day.

You're right when you say Silent Circle should be scrutinized and criticized.

Nevertheless, I disagree when you imply that the unavailability of its source code is a show stopper. Source code only makes one small part of the security audit a little bit easier.

Security is a process, not a feature.