There is a further benefit to the idea of TBird (for example) quietly encrypting email to/from encryption enabled correspondents.

With the current low percentage of encryption enabled email, encrypting your mail is like waving a big red banner that says, "Keep an eye on me! I'm a dangerous intellectual (at the very least)". Quietly, and innocently, increasing the percentage of encrypted traffic, would get us closer to the same traditionally legitimate privacy of the paper envelope.

This idea sounds like a good candidate for a Mozilla labs project. You may wish to consider adding it to the "Labs Ideas" page (http://wiki.mozilla.org/Labs/Ideas).

I applaud the design simplicity you describe. Its like the simplicity in Key2Mail, or corproate email security solution.
However, we even avoid the PKI processes, yet still maintain end to end email confidentiality/assurance, while allowing the corporate entity (not the individual) to maintain control of the business' email content, something your model doesn't permit (not does PGP, S/MIME et al).

Cheers,
Lyal