The internet connection didn't magically fix itself while I was out. I think I've got my work cut out for me tomorrow. My first guess? The modem somehow got fucked up. I've checked the firewall and everything seems fine with it. Both the firewall as well as the modem were restarted (power down, not just a reset) and I still can't get anything beyond an IP assigned by the DHCP server in the firewall.
First work tomorrow is replace the DSL splitters, though I'm quite certain those are not the problem - they are just the easiest to replace. After that, I'm taking the firewall out of the loop and connecting the modem directly to the main box to see the result of that. If the modem is fucked, I'll know it right away.
Anyway, if you don't hear back from me right away (IM, mail, comments, whatever) it's probably due to a very limited connection. I don't feel like sending unsecured data over a network that ain't mine, and is open to everyone who happens to feel like connecting.