The plan:

September 5, 2014 Trunk is open for business. (Post 4.0)
September 29, 2014 Initial 4.1 meeting.
October 1, 2014 Decision time for features being developed as a plugin. Merge window opens for any blessed for 4.1.
October 15, 2014 (+2w) Feature plugin merge window closes.
October 29, 2014 (+3w)
November 5, 2014
November 13, 2014
November 14, 2014
Beta 1.
From this point on, no more commits for any new enhancements or feature requests in this release cycle, only bug fixes and inline documentation. Work can continue on enhancements/feature requests not completed and committed by this point, and can be picked up for commit again in just a few weeks at the start of WordPress 4.2.
November 5, 2014 (+1w)
November 20, 2014
Beta 2.
November 12, 2014 (+1w)
November 26, 2014
Beta 3.
November 19, 2014 (+1w) Beta 4.
November 24, 2014 (+1w)
December 1, 2014
December 11, 2014
Release candidate. (More release candidates as necessary.)
Week of December 8
December 16, 2014
December 18, 2014
Target date for release of WordPress 4.1.

To get involved in WordPress core development, head on over to Trac and pick a 4.1 ticket. Need help? Check out the Core Contributor Handbook. Get your patches done and submitted as soon as possible, then drum up people to test the patches and leave feedback on the ticket. Patches for enhancements won’t be committed after the posted dates, so that we can all focus on squashing bugs and hopefully deliver the most bug-free WordPress to date. Wish us luck!