Spring 2.5 test classes (eg those that use @RunWith(SpringJUnit4ClassRunner.class) ) implicitly rely on a class which was removed in JUnit 4.5
See this Jira: http://jira.springframework.org/browse/SPR-5145
Unfortunately, the latest Eclipse (3.5) ships JUnit 4.5 by default, meaning that the newly-production Eclipse version is broken for running tests written for the current production Spring version.
No workaround is currently available (and "use Spring 3.0" as a resolution isn't very helpful for production stuff - it's still in beta) - I will investigate with the Eclipse people as to whether anything (eg a downgrade of the JUnit plugin to 4.4 would be feasible) but don't hold out much hope.For now, looks like it's back to running Spring tests from the command-line with a local JUnit 4.4 jar on your classpath.
(Btw, if anyone does have a good workaround, please let me know.)