Daniel Cousineau has posted a quick guide to migrating database information with Doctrine:

I was talking with someone [...] here at ZendCon and discovered that they were having trouble with migrations in Doctrine. Having gone through the same issues of Doctrine seemingly not being able to figure out your changes and generate migration classes, I thought I’d post the solution here for future reference.

It's four quick steps that'll get Doctrine to automatically generate the differences (deltas) and upgrade your models to reflect these changes.