Sometimes my brain and I fall out, and we refuse to communicate. Yesterday was a day like that.
Having put my TV computer back together again (see post), I realised that I hadn't installed conky or guake on it. So I did. Having edited my default conky.conf (a bit like the one in this post) to change eth0 to eth1, I was wondering why my wireless card wasn't showing up in the ifconfig -a output. lspci showed that I had a broadcom chip, so I installed broadcom-sta-dkms. Nothing much happened, so I rebooted. And...no network card whatsoever recognised (including the ethernet one).
Face palm. I don't have a wireless pci/pci-e card on that computer. I should know -- I had just rebuilt it...
Oh well. apt-get autoremove broadcom-sta-dkms. Nothing. Reboot. Still no eth card -- also, the eth card was obviously the broadcom one (broadcom 4401). But why wasn't it working? I had uninstalled the broadcom package after all.
A bit of googling suggested loading b44 so modprobe b44 it was. Still nothing. Then a thought hit me -- ls /etc/modprobe.d/*blacklist*
And yes -- the broadcom package had installed a broadcom-wl-blacklist.conf file in /etc/modprobe.d -- and b44 was blacklisted. At this point I could either edited the file and comment out b44, delete the file entirely or purge the package and everything would be back to normal. So I did.
So remember: just because you uninstall a package (apt-get autoremove) doesn't mean that you've restore your system to the state it was before you installed the package. To at least have a chance of restoring it you need to purge your package so that settings files are removed too (apt-get purge).