I have set up Jenkins, but I would like to find out what files were added/changed between the current build and the previous build. I'd like to run some long running tests depending on whether or not certain parts of the source tree were changed.
Having scoured the Internet I can find no mention of this ability within Hudson/Jenkins though suggestions were made to use SVN post-commit hooks. Maybe it's so simple that everyone (except me) knows how to do it!
Is this possible?
The CI server will show you the list of changes, if you are polling for changes and using SVN update. However, you seem to want to be changing the behaviour of the build depending on which files were modified. I don't think there is any out-of-the-box way to do that with Jenkins alone.
A post-commit hook is a reasonable idea. You could parameterize the job, and have your hook script launch the build with the parameter value set according to the changes committed. I'm not sure how difficult that might be for you.
However, you may want to consider splitting this into two separate jobs - one that runs on every commit, and a separate one for the long-running tests that you don't always need. Personally I prefer to keep job behaviour consistent between executions. Otherwise traceability suffers.