On DZone.com today there's a new article from Giorgio Sironi about making the switch from Doctrine 1 to Doctrine 2 and some of what might be involved.

Doctrine 2 is an implementation of the Data Mapper pattern, and does not force your model classes to extend an Active Record, nor to contain details about the relational model like foreign keys. [...] Note that you will have to run your application of PHP 5.3 for Doctrine 2 to work, mainly because of the use of namespaces in it.

He talks about some of the other differences including maintaining PHP classes and the metadata in them rather than just a YAML schema to map to your database. There's also a difference in how to interact directly with the Doctrine engine. Direct access has been replaced with a dependency injection approach.