I noticed that SVN triggered a build that was not necessary:

Directly after creating the project, SVN triggered a build, since there was no workspace. The build was successful:

Build #1

Build was triggered by SCM modification.
Building on Slave CruiseControl in workspace C:\.jenkins\workspace\QUIPSY Database Update
Checking out a fresh workspace because there's no workspace at C:\.jenkins\workspace\QUIPSY Database Update
Cleaning local Directory .
Checking out svn://svn/QUIPSY4/DatabaseUpdate/trunk at revision '2013-02-09T18:06:45.390 +0100'
...
At revision 22781

Some minutes later it triggered again, without any reason, since there was no SCM change – look at the revision number (22781), it is unchanged!

Build #2

Build was triggered by SCM modification.
Building on Slave CruiseControl in workspace C:\.jenkins\workspace\QUIPSY Database Update
Updating svn://svn/QUIPSY4/DatabaseUpdate/trunk at revision '2013-02-09T18:20:55.189 +0100'
At revision 22781
no change for svn://svn/QUIPSY4/DatabaseUpdate/trunk since the previous build

(translated log messages back from German L18N)

While it does not real harm, it is not nice that I spend money in electric power for builds that make no sense!