Mule 2 is already very Spring-oriented, or at least very Spring-friendly. Will Mule's distribution, which is currently a nice set of Maven-build artifacts, evolve to a set of OSGI bundles a la Spring for version 3?
If MuleSource follows that path and distributes Spring Dynamic Modules, that would allow people to run on the SpringSource dm Server, if they want it, and benefit from sweetnesses like hot service replacement.
I doubt that Mule could use SpringSource dm Server as a default platform unless they clear up what I think could be licensing incompatbilities. But end users could make that choice and put the two Sources, Mule and Spring, together.