After the recent additions of Hibernate, Ehcache and Solr (Lucene) to ColdFusion 9, it's starting to become really clear what ColdFusion does best: provide a clean and easy interface to powerful, open-source Java libraries.
With that being said, I would vote for Spring to be included next. While ColdSpring is great, why not go straight to the source? Not only would it allow for a tighter, seemless integration between ColdFusion and Java objects, but it would also leverage the power and performance of Java for our IoC and AOP needs. On top of that, Spring provides other services, such as integration with BlazeDS for Flex powered applications.
Just think, you could configure Spring inside your Application.cfc using this.iocEnabled and this.iocSettings.iocConfig. And if you didn't want to use XML, you could define your beans using annotations in your components (component beanid="userService" scope="singleton") and wire in your dependencies through properties (property name="userService" autowire="true";). It could look pretty similar to the way Hibernate was implemented.
Granted trying to integrate Spring into ColdFusion is probably easier said than done, but if they got it to work with Hibernate, I'm sure it could be done with Spring too.