After reading this article a few days ago, I realised that not everyone who wants 8-bit + alpha PNGs has Fireworks (or wants to launch Fireworks just to re-save an image), myself included. There were plenty of instructions online about how to install server-side components or tweak GD or ImageMagick to do it but that all seemed like hard work.
A quick bit of googling and copy-pasting later and this is the outcome. A shiny web service you can drag 24-bit + alpha PNGs into and get 8-bit + alpha PNGs out of. If you need the service, this'll be very handy, if you don't, this'll be quite pointless. There's not much middle ground.
The code's available on github if anyone wants to have a look at it or improve it.