The PHP 5.4 MVC with Shoes On: ActiveRecord, User/Session, Generators, SCSS, (Twitter Bootstrap) Integration, Workers, Cron Management, Image Manipulation, Caching, Git Management, Mail & Mail Parsing, OCR, Scraping, Selenium, Mustache, Markdown, Phone & Text Messaging, WebSockets, BitTorrent and much more!
https://github.com/lunaru/MongoRecord
Missing a lot of the features PHPActiveRecord provides, but could potentially be adapted.. maybe just write a mongodb adapter for PHPActiveRecord?