What's the difference between ServiceMix and FUSE?
IONA FUSE ESB is based on Apache ServiceMix to deliver a platform to service-oriented integration. As a point of comparison, consider the Java EE runtime contained in an application server, but think much, much lighter and more agile than requiring a full Java EE stack everywhere. FUSE is designed to provide a complete runtime for developing applications for handling service-oriented integration.
Consider what Red Hat has done with Linux. Anyone can build their own Linux distribution (http://www.linuxfromscratch.org/) but few people care to do this because of the effort involved in the maintenance, support, testing, validation, etc. of such an effort. Red Hat provides the world with an easily installable Linux distribution with much value added and enterprise level services and support. FUSE is very similar to RedHat Linux insomuch as adding value to quality open source software (Apache ActiveMQ, Apache Camel, Apache Derby, Apache ServiceMix, Jetty, etc.) and providing services, support and training for a vetted version of the software.
For some additional insight into the FUSE model and how it works, see my blog entry entitled The IONA FUSE Model and It's Benefits to the Community. You can also find additional information about FUSE ESB and the Open.IONA community at "Why join our community".