This question came up on twitter and it wasn't a quick answer so thought I'd answer here.
So right now fabric8 is aligned with the JBoss Fuse 6.1 release thats coming out soon which is based on Apache Karaf 2.3.x (along with Apache Camel 2.12.x and Apache ActiveMQ 5.9.x).
We've lots of production customers on Fuse 6.0; so are not yet ready to take a major update of Karaf (3.x) which has lots of major changes in it and we've not yet put it through our QA process. (3.0.0 hasn't been out 2 months yet so its still pretty new).
As soon as the JBoss Fuse 6.1 release is out, fabric8 will move to support Karaf, Tomcat, WildFly containers (hopefully vertx and jetty too) along with stand alone JVMs like DropWizard and Spring Boot - in addition to its current support for managed processes, OpenShift and jclouds.
We've migrated fabric8 to be completely based on Declarative Services and moved away from OSGi blueprint; as its leaner, meaner, avoids dynamic proxies and is easier to deal the asynchronous modular nature of OSGi as things are dynamically added and removed. An added benefit is the use of Felix DS annotations lets us auto-generate nice stand alone OSGi tooling and fabric8 based fabric-aware tooling in hawtio for all of the services in fabric8).
Ideally it'd be good to migrate fabric8 straight to Karaf 4.x (or whatever it ends up being called) thats based on pure Declarative Services; as it'd make the OSGi based distribution of fabric8 leaner & meaner. But we'll see in a couple of months how things are looking in Karaf-land.
We're very excited about fabric8 being completely poly-container; so we can use fabric8 to provision and manage Karaf, Tomcat, Wildfly & vertx containers; stand alone JVMs along with working great with docker, OpenShift and jclouds - all with a lovely hawtio console for everything! Its going to totally rock! :)