I verified this to be a problem as well with the following configuration:
Eclipse 3.1M6
Derby Core plug-in, version
IBM Integration plug-in, version 1.0.0
As you suspected, the com.ibm.cloudscape.ui.launch.StartDerbyServerLaunchConfigurationDelegate. class
is in the ui.jar file contained in the IBM Integration plug-in.  I verified this class is in
my plug-ins directory, so I don't think that is the problem that Xavier is experiencing.
Nor has it been a requirement in the past that this jar file be in the classpath for the
project.  All of the jars mentioned are correct.
Something must have changed in 3.1M6, because as far as I can remember the above
combination of plug-ins worked okay on 3.1M5, although I will verify this.

Philip Wilder <050503w@acadiau.ca> wrote:

The name of the missing class would make me suspect that it should be
found in the derbyui.jar. While I don't think this should need to be on
your classpath list couldn't hurt to try. If it seems the class in
question is missing from the derbyui.jar file is it possible your file
is corrupt or out of date?