After installing the Conditional BuildStep Plugin it works fine. However, if you restart Jenkins it will fail to startup. The log contains the following:

May 2, 2013 9:23:45 AM jenkins.InitReactorRunner$1 onTaskFailed
SEVERE: Failed Finalizing set up
java.lang.NullPointerException
at org.jenkinsci.plugins.conditionalbuildstep.ConditionalBuilder.buildDependencyGraph(ConditionalBuilder.java:172)
at hudson.util.DescribableList.buildDependencyGraph(DescribableList.java:213)
at hudson.matrix.MatrixProject.buildDependencyGraph(MatrixProject.java:773)
at hudson.model.DependencyGraph.build(DependencyGraph.java:90)
at jenkins.model.Jenkins.rebuildDependencyGraph(Jenkins.java:3585)
at jenkins.model.Jenkins$19.run(Jenkins.java:2566)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:893)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
May 2, 2013 9:23:45 AM hudson.WebAppMain$2 run
SEVERE: Failed to initialize Jenkins
org.jvnet.hudson.reactor.ReactorException: java.lang.NullPointerException
at org.jvnet.hudson.reactor.Reactor.execute(Reactor.java:246)
at jenkins.InitReactorRunner.run(InitReactorRunner.java:43)
at jenkins.model.Jenkins.executeReactor(Jenkins.java:904)
at jenkins.model.Jenkins.<init>(Jenkins.java:804)
at hudson.model.Hudson.<init>(Hudson.java:81)
at hudson.model.Hudson.<init>(Hudson.java:77)
at hudson.WebAppMain$2.run(WebAppMain.java:214)
Caused by: java.lang.NullPointerException
at org.jenkinsci.plugins.conditionalbuildstep.ConditionalBuilder.buildDependencyGraph(ConditionalBuilder.java:172)
at hudson.util.DescribableList.buildDependencyGraph(DescribableList.java:213)
at hudson.matrix.MatrixProject.buildDependencyGraph(MatrixProject.java:773)
at hudson.model.DependencyGraph.build(DependencyGraph.java:90)
at jenkins.model.Jenkins.rebuildDependencyGraph(Jenkins.java:3585)
at jenkins.model.Jenkins$19.run(Jenkins.java:2566)
at org.jvnet.hudson.reactor.TaskGraphBuilder$TaskImpl.run(TaskGraphBuilder.java:146)
at org.jvnet.hudson.reactor.Reactor.runTask(Reactor.java:259)
at jenkins.model.Jenkins$7.runTask(Jenkins.java:893)
at org.jvnet.hudson.reactor.Reactor$2.run(Reactor.java:187)
at org.jvnet.hudson.reactor.Reactor$Node.run(Reactor.java:94)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)