Would be great to trigger builds from perforce instead of having to poll. Current url trigger mechanism in Jenkins is not realistic because you would need to create new perforce trigger for every job.

[1] I guess you need a single entry point where the change-number is passed in. Then the P4 plugin could then work out which Jenkins job or jobs to trigger?

or ...

[2] A subscribe model, A bit like the old p4 review/reviews command, but for Jenkins jobs to subscribe to Perforce depot paths.

Design 1 is similar to what the Subversion plugin does. To use this with Subversion, the Subversion server is configured with a post-commit hook that makes a request to Jenkins over HTTP. One significant difference from your design is that the request also includes the changed paths as POST data, removing the need for Jenkins to ask the Subversion server for changes. See the Subversion plugin's wiki page for more details.

I like the POST data idea for change-list details. I may combine [1] and [2], using a web service for Jenkins to subscribe to and then use Perforce triggers to poke the web service.

[Perforce]                [Web Service]               [Jenkins]
     |                          |                         |
     |                          | <-----(subscribe)------ |
     |                          |                         |
     | ----(change-commit)---> [ ]                        |
     |                         [ ] ----(POST change)---> [ ] 

Would this be a standalone web service, needing to be hosted outside Jenkins and Perforce? I'm no expert in the inner workings of Perforce so this might be the best solution, but requiring a third piece of software to be hosted is a drawback to me.