I checked out the TOC for JBoss in Action and one of the items that gave me pause (made me go 'Hmmm') was mention of the "JBoss Microcontainer". As I really hadn't heard about this previously (I primarily work with WAS, Tomcat, and BEA), I did a quick google search and came up with this very brief write-up:
The Microcontainer is an IoC container. It is the backbone of the JBoss 5.0 architecture and all of the JBoss services are either already configured using the Microcontainer or are planned to be in future versions of JBoss.
I'm not sure what would make it any more "special" than Spring in particular because I've never used it as a standalone IoC container. From a JBoss perspective, I guess what makes it special is that you can use it to wire you application together, then wire it into JBoss. That being said, there is also a Spring deployer in JBoss - which I haven't played around with yet - that is supposed to let you integrate your Spring-configured application in JBoss.