[blowski]

Makes sense from Google's POV. Separating 'Gmail the email client' from 'Gmail the IMAP/POP/SMTP server', the latter requires a lot of investment with not much return. It seems that most of the value to Google comes from you using their client, since they can:

* show more contextual advertising

* track you as you travel around the web (since you're more likely to be logged in)

* use other Google services - especially Google+

So by adding proprietary features that only mean anything when viewed within Gmail itself, it's a big win for them. Also, once people become used to the microdata, they can gradually reuse it in other Google services. For example - complete a Google form straight from an email, that then does something special on an App Engine product.