There is a new white paper that has just been published. It provides an overview of reasons why you would want to update to SP2, some considerations to think about before updating, the process of updating, and an FAQ and troubleshooting guide for installation issues. You can find the white paper by clicking here.
This is a really good document to read so that you can carefully consider some of the branching decisions for runtimes you want to build. For example, if you are currently building and maintaining XP Embedded RTM or SP1 runtimes and you want to update your database to SP2, you will need to maintain a separate database if you want/need to continue providing updates to your SP1 runtimes without also updating them to SP2.