PostgreSQL 9.0 beta 3 will be wrapped in the next few days, and at the same time, we'll be branching the tree to begin 9.1 development. This is a new thing for us. In the past, we've waited until the previous release was shipped before opening the tree to new development. However, at the PGCon 2010 development meeting, we decided to try something different this time.
I believe that the primary motivation for this change was that, as we get closer to release, there are fewer and fewer issues to work on, and fewer and fewer people who can be involved in fixing them. So, waiting until release to branch the tree leaves a substantial portion of the developer community sitting idle. A second advantage is that it shortens the time between releases - our tentative plan is to use the same release schedule for 9.1 that we did for 9.0. The first CommitFest for 9.0 began on July 15, 2009, and the first CommitFest for 9.1 will begin on July 15, 2010; the last CommitFest for 9.0 began on January 15, 2010, and the last CommitFest for 9.1 will begin on January 15, 2011. Of course, the actual release date will almost certainly be different, but the plan is for feature freeze to happen about the same time next year that it did this year, so that we can continue to have releases about a year apart.
Of course, the danger of concurrent development is that the work people are doing for 9.1 may distract us from finishing 9.0. Hopefully that won't happen, because I think there is a lot to like about the new process.