There is a very helpful mailing list that you can subscribe to for instant notification of any "commits" to the joomla 1.6 development trunk but there is nothing better than doing a full checkout.
If you're not familiar with an SVN repository then you will need some software to help you, personaly I use TortoiseSVN and then do a checkout of:
http://joomlacode.org/svn/joomla/development/trunk/
A quick scan of the recent commits show several things that third party developers should be aware of. I won't go into detail here as a personal check is the best thing to do but one thing to keep an eye out for is for files moving location as the framework becomes separated from the cms.
There are also three new public mailing ists that developers can contribute to.
With open SVN and public mailing lists there is no excuse for any developer not to become involved in the future of joomla.