Some folks here at Yahoo! have used the Phing plugin in the past and ran into trouble trying to use it in a master/slave configuration. Here is the exception they were getting (this stack trace is from over a year ago):

java.io.IOException: Unable to serialize hudson.plugins.phing.PhingInstallation$1@1cb3087
at hudson.remoting.UserRequest.serialize(UserRequest.java:126)
at hudson.remoting.UserRequest.<init>(UserRequest.java:60)
at hudson.remoting.Channel.call(Channel.java:475)
at hudson.plugins.phing.PhingInstallation.getExecutable(PhingInstallation.java:72)
at hudson.plugins.phing.PhingBuilder.perform(PhingBuilder.java:120)
at hudson.model.Build$RunnerImpl.build(Build.java:193)
at hudson.model.Build$RunnerImpl.doRun(Build.java:144)
at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:272)
at hudson.model.Run.run(Run.java:865)
at hudson.model.Build.run(Build.java:112)
at hudson.model.ResourceController.execute(ResourceController.java:93)
at hudson.model.Executor.run(Executor.java:113)
Caused by: java.io.NotSerializableException: hudson.Launcher$RemoteLauncher
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1156)
at java.io.ObjectOutputStream.defaultWriteFields(ObjectOutputStream.java:1509)
at java.io.ObjectOutputStream.writeSerialData(ObjectOutputStream.java:1474)
at java.io.ObjectOutputStream.writeOrdinaryObject(ObjectOutputStream.java:1392)
at java.io.ObjectOutputStream.writeObject0(ObjectOutputStream.java:1150)
at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:326)
at hudson.remoting.UserRequest.serialize(UserRequest.java:123)
... 11 more

I've attached a patch that fixes this by determining the executable name on the master before starting the remote call and removing the launcher from the callable state.