DISCLAIMER: THIS ANSWER CONTAINS MY OPINION AND SOME FACTS
but to me it looks like it is becoming the latest 'de facto' mail protocol for email/calendar/contact sync'ing due to the iPhone's support for it.
Not really, the 'de facto' mail protocol is the MSMAPI protocol for Exchange. I would argue that for mail/calendar/contacting syncing this is the most used protocol. Also implementing it depends on Microsoft licensing it to you. Just because Apple can do it doesn't mean you can too.
For mobile devices I would think that ActiveSync and Blackberry SRP are the two most popular 'standards'
For external users, you want to use Outlook Anywhere or Outlook RPC/HTTP.
As for getting Lotus Notes into Outlook, use the connector.
This solution would probably eliminate the need for all the hacks needed to connect outlook to 'X' mail (Gmail, notes, etc)
The connector APIs are not hacks, they are defined APIs for people to extend the functionality of Outlook. You can also write your own MSMAPI protocol if you want to.