In a recent post to his blog Grzegorz Godlewski talks about some of the criteria to consider when selecting your source code management tool (between Subversion, Git and Mercurial).

Each of them has its own advantages and disadvantages so the project’s requirements, development environment and team members receptivity should be taken into consideration. Relying on my own experience in project management and using version control systems to manage the development process I’ve formed the conclusions listed below that may be helpful while considering usage of one of described SCM’s in your own projects.

He talks about how the size of the project can effect the decision and a bit more detail about what each of the version control systems are good at. The PHP project itself has recently made the shift to git away from Subversion (you can find more information on the PHP.net site).

On the EllisLabs site there's a recent post talking about some of the big changes that've happened to their development process surrounding CodeIgniter and its future version.

EllisLab today announces changes to our internal development processes, including dropping Subversion in favor of Mercurial and adopting Assembla as our agile software development management tool. Along with these changes, CodeIgniter 2.0 pre-release code is in development, and is now hosted at the Mercurial-focused social coding site BitBucket.

They also talk about their move to the Scrum method of software development and how this (and their change of version control system) can help the future of CodeIgniter (version 2.0) come even sonoer through contributed updates and bugfixes made much simpler through the features of BitBucket (similar to Github). If you'd like to contribute, you can follow the CodeIgniter project and get busy contributing back to this great framework.