Good question!
In regards to testing your integration logic in insolation, you will find that Spring Frameworks already rich support for unit-testing is supported within SI. Thus,
Junit works, and so does ezmock/mockito or whatever is your favorite test suite. Because SI is indeed Spring, you can also work with the junit4runner to test your logic in insolation. Of course because the SI are integration apps, this means having more resources available so you would want to be confident that ad-hoc stubs, or real 'integration' test resources meet your testing requirements.
So, Spring Integration is just Spring. business as usual, otherwise.