This week I'd been fighting with a Chef install to do something relatively simple. Bogged down in a rats nest of complexity (extra Ruby scripts referencing Chef environment files, etc etc), I decided to see if there was a easier way.
Although I wasn't actually trying to install Jenkins, my workflow led me to look at installing it with Ansible as a proof of concept. Basically I wanted to see how easy this task could really be.
I've put the playbook, along with some notes, into a Github repo, which you should feel free to fork and play around with.
Currently this is a very simple example - spin up the EC2 instance, then install and run Jenkins as its own daemon (on the default port of 8080).