One topic that has come up in the past is PHP 5.3 compatibility. Many other frameworks are either requiring PHP 5.3 already, or they are moving to rely on PHP 5.3-only tools like Composer. PHP 5.3 provides a number of features that make it easier to write clean code (such as late-static binding, namespaces, and closures).
One of the key goals for SilverStripe - and the reason that we kept PHP 5.1 support for so long - was that it could be run on the PHP packages provided by major distributions. RHEL/CentOS ran PHP 5.1 for a long time, but that is no longer the case - RHEL6 runs PHP 5.3, as to the most up to date releases of all of the distributions that I investigated.
However, PHP 5.2 still has 3x as many deployments as PHP 5.3. Based on that, it seems like dropping PHP 5.2 support would make many users unable to use SilverStripe.
But this is all speculation. To get some concrete data, we've put together a survey that we would like as many people to fill out! On it, we ask a few questions about whether people still need PHP 5.2 and/or whether they would benefit from PHP 5.3.
On the basis of this survey, we can make an informed decision about when and whether it would be appropriate to drop PHP 5.2 support. It would be great if you could all fill it out!
The survey is here: http://www.surveymonkey.com/s/GXBJ283
Once we have the data from the survey, we'll post back here with the results and our plans for PHP 5.2 support. The goal is to make a decision that is in the best interests of the whole community, so this is your time to be heard. :-)