Ah Steam, my old nemesis - resolved.

[Dr_BenD_over]

I had re-enabled Windoze firewall at some point, but that was the first thing I killed.

I've got a bunch of stuff to try when I get home, I think the first will be booting into Vista since I had never setup a dial-up connection in that and see if the problem persists. Then try installing Steam on the Atom and bypass the router altogether. If that works I'll disable all the ports I've forwarded for steam and try that through the router.

[Dr_BenD_over]

So,

Boot in to Vista, Steam connects, downloads update, subsequently won't connect again.

Install Steam on Atom, Steam connects, downloads update, won't connect again

Bypass router, login screen comes up, can't remember password, hunts for the scrap of paper I wrote it on, enters it, connects to Steam.

Turn off all ten or so port forwards I've setup for Steam since like back in the beta days (it's a rather old router model and it's my second of that type) and sure enough Steam works. I'm guessing some of the ports that used to have to be manually forwarded have been phased out with proper uPnP support. I don't know why Valve still has that port forwarding page up in their troubleshooting section.

[On2wheels]

Figures. Steam has become quite user friendly the past year or 2, I rarely have problems. The Cloud is quite impressive too, saving my old savegames from 2yrs ago.

[Varroa]

I have never used portforwarding for steam EVERY and I have been on it since something like 2006 or maybe earlier.

[Dr_BenD_over]

I've been using it with the same model router since the first public beta in 2002. A lot of my port forwarding probably goes back to then or further.