The JSR 208 JBI specification doesn't directly address the issue of having JBI span more than a single process.
The Open ESB implementation of JBI supports homogeneous distribution in the form of Glassfish clusters.
The Fuji project adds support for heterogeneous distribution in the form of the Proxy Binding bundle. This recently hatched component will allow for experimentation in heterogeneous distribution as well as exploring the benefits and limitations of either form of distribution. Exploring how best the different forms of distribution can be mixed should be an interesting topic.
The following links contain more detailed information: