I think many of the high-level concepts would carry over from your BizTalk experience. I would recommend spending a bit of time with the Enterprise Integration
Patterns website (or better yet, the book itself):
http://www.eaipatterns.com
You can explore the "Integration Patterns" on the right hand column there.
Then, if you go through some basic Spring material, mainly to get the hang of dependency injection,
you should be able to start working with Spring Integration without too much trouble. I'd recommend this blog:
http://blog.springsource.org/2010/11/09/green-beans-putting-the-spring-in-your-step-and-application (that's the first in the 'green beans' series).
One of the goals of the framework is to allow you to keep your Java code as simple as possible - and most importantly, focused on business logic rather than integration concerns which are instead handled by the framework.
Hope that helps.
-Mark