Hi Christian,
I suspect this is a load order problem. After the release of 0.9.6 [1], Mocha no longer loads Test::Unit itself. Instead Mocha now needs you to load Test::Unit *before* you load Mocha. Exactly how you solve this depends on how you are currently loading Test::Unit and Mocha, but usually it may simply be a matter of moving a require 'mocha' statement below a require 'test/unit' statement.
If you need any more help, please post to the Mocha mailing list [2].
Thanks, James.
[1] Mocha Release 0.9.6 - James Mead [2] http://groups.google.com/group/mocha-developer
