Apparently the upgrade to php 5.5.12 changes the default mode for the php-fpm socket files from 666 to 660. A direct result of this is that nginx is no longer able to access these files. In other words; shit gets broken.
At first I added the php-fpm ‘listen.mode = 666’ line to the php-fpm configs which fixed my problem. Later, however, a colleague of mine pointed out that php-fpm also has a ‘listen.owner’ setting. I removed the ‘listen.mode’ line and added ‘listen.owner = nginx’ (because nginx runs as the nginx user), the php-fpm socket files were again chmodded at 660 but because they were now owned by the nginx user this was no longer a problem.
As Sterling Archer would say; “Problem-o solved-o!”.