9.0.6 is anticipated soon, but I don't think there's a solid date to share right now.
An educated guess might be next week:
•9.0.5 | Released: 8/24/15
•9.0.4 | Released: 7/21/15
•9.0.3 | Released: 6/23/15
•9.0.2 | Released: 6/2/15
v9.0.6 will be out very soon, tentative ETA is sometime this week or early next week.
Sounds as though new versions of 9.0.x will continue for the time being. Release of 9.0.6 is likely before the end of September. An exact date is not critical.
Will look forward to testing 9.1.x in November/December.
Is 9.1 a continuation of 9.0.5 with additional features? This was the case with 8.2.x versus 8.3.x with Kerberos support.
Yep, v9.1 has some cool new features. From http://onlinehelp.tableau.com/current/server/en-us/help.htm#whatsnew_server.htm:
The following features and enhancements are new in Tableau Server 9.1.
We just got on 9.0.5 a couple weeks ago. And I concur about not being on bleeding edge. Therefore, we're waiting until 9.1.2 as we don't want to have a release called 911
Yes, we don't want 911 either.