Updated: 16/09/2012 1:18pm
The move from SilverStrip 2.x to version 3.0 is a big one! Not only has the CMS improved dramatically, but the internal workings have changed as well. Lots of emphasis now lies on SilverStripe being an actual framework. So much so, that the sapphire directory is now actually called 'framework', although you'll find on GitHub the module is still called sapphire.
Some of the objects we used to know so well now have different names: the trusted DataObject is still there, but the DataObjectSet has been replaced by the DataList, to name only one. There's the introduction of the GridField (rendering the ComplexTableField obsolete) and lots of config settings, as well as the language files have now been transferred to yaml. The templating engine has seen some significant changes as well: the old control structure has been divided in to two new structures: 'loop' to iterate DataLists and 'with' to acces the scope of a single DataObject.
In the mean time a great amount of effort has been put in updating the official SilverStripe documentation, and it's growing steadily. So although upgrading a site to version 3 can be a a somewhat daunting task, these pages will be of help:
At this moment version 3.0.2 is nearing stable, and daily new updates, ideas, suggestions and pull request are submitted. One thing's for sure: SilverStripe 3 very much alive!