I had the exact same issue you did. I've just gotten past it by installing a 1.6 JRE on the slave (upgrading it from a 1.5 JRE). Problem went away. I presume either Hudson or one of the plugins we use (SLOCCount Plugin, Google Calendar Plugin, Doxygen Plugin, Warnings Plugin, Jabber Plugin, NAnt Plugin) has a dependency on 1.6.
For reference I'd also upgraded to the latest Hudson version (1.347) from 1.339. Both versions had the same problem.
To install the Java system (on Ubuntu 8.04) I used: