Loaded the old config, removed all the NAT rules, re-created them like I just did now… still doesn't work. So wierd. I'm now restoring pieces of my original configuration onto the working config.
Update: Alright, so I've restored the bits of the configuration I didn't want to have to re-enter (aliases, rules etc.) and now things are working fine again. I can only assume that something was messed with my configuration file. I've compared both configurations, and while I can't see any gaping issues, there are various discrepancies, mostly in the formatting of the XML. I wonder if my configuration became corrupt, or if there are some small changes between 1.1 and 1.2 that I got caught up on.