How to "Group By" with MongoDB
Sometimes you may want to get an array of distinct values in a collection. You can accomplish this using the distinct() method:
$ages = $dm->createQueryBuilder('User')->distinct('age')->getQuery()->execute();
There's some detailed documentation on the Group command on their website.
There are some examples of Group using the PHP libarary here.
If your data set is really big, you may need to look at running map-reduces to "pre-group" this data.
New answer using Mongo aggregation framework
After this question was asked and answered, 10gen released Mongodb version 2.2 with an aggregation framework. So the new best way to do Group By is with the $group operator. In PHP use MongoCollection::aggregate.
FYI, 10gen also has a handy SQL to Mongo Aggregation conversion chart worth bookmarking. It has examples of all the common SQL queries that use SQL aggregation and how to translate them to Mongo.