Jonathan Wage posted a teaser of Doctrine 2.0 running on PHP 5.3:
Something we haven’t talked to much about is Doctrine 2.0! Roman has been quietly working on the next major major version of Doctrine, 2.0. This version will be taking a big leap by requiring PHP 5.3! The progress this new version of PHP is so great and it lends itself very well to the world of Object Relational Mappers.
Of note, it looks like memory usage is down and speed is up:
The Doctrine test suite uses 31% less memory and is 17% faster when running under PHP 5.3!
(Via the Doctrine Blog.)