Vibration stopped working on Galaxy S2
This turned out to be a hardware fault which is pretty common on the SGS2 units. The vibration device is mounted next to the audio jack, meaning that it sometimes fails if the jack has been stressed. There's a short video (warning: nsfw language) that describes the issues and how to fix them.

Essentially it works as following:


Insert a cable into the headphone jack.
Lightly press on the jack, in the direction of the front-facing camera (left, if looking at the phone from the front)
Whilst lightly pressing in this direction, remove the jack.
Vibration should be restored... for now.


The long term fix is to get the vibration motor re-mounted by a qualified repair technician.