I didn't want this to get lost since JENKINS-29979 was closed out.
The P4 Trigger isn't firing a Pipeline job off when a POST request comes in per the /p4/ help page in Jenkins, or if you do a manual trigger. I do see the request come in, after which P4 probes the Pipeline projects, but then nothing happens:
Jun 02, 2016 11:36:16 AM INFO org.jenkinsci.plugins.p4.trigger.P4Hook doChange Received trigger event: {change:123456,p4port: "p4host.example.org:1666"} Jun 02, 2016 11:36:16 AM INFO org.jenkinsci.plugins.p4.trigger.P4Hook probeJobs P4: probing: pipeline-project-01 Jun 02, 2016 11:36:16 AM INFO org.jenkinsci.plugins.p4.trigger.P4Hook probeJobs P4: probing: pipeline-project-02 Jun 02, 2016 11:36:16 AM INFO org.jenkinsci.plugins.p4.trigger.P4Hook probeJobs P4: probing: pipeline-project-03
For now, we are doing a poll every couple of hours, but it would be great to use the Trigger mechanism in the future.