If you're a Mavenite Spring 2.5 user, you'll need to jump through some extra hoops to use Spring Security 2, just as you had to when using Acegi with Spring 2.0.
First off you'll want to exclude org.springframework:spring-remoting and org.springframework:spring-support, as these two artifacts no longer exist in Spring 2.5.
In addition, you will most likely need to include org.springframework:spring-aop and org.springframework:spring-orm as dependencies, unless you're already using them.
That said, Spring Security's new config is nice. One particular security config file went from 118 lines to 24!