Grr, the PCBweb mentioned is Windows only! Requires silverlight, and not even that, "Sorry, Linux based operating systems using Microsoft's Silverlight are currently not supported." (can Linux even use Silverlight in the first place? even more vague.)
It does let you sign up and give them your info though, before it tells you
Well, not that it matters, or it's meant to be like that.
I guess I can't complain, but at least there is Eagle for Linux and others emulate alright too. It's a friggen web app! I guess Dave and Chris were right that it could possibly work offline if it's a chunky silverlight application, like flash usually, unless it's got something funky to tie it to the domain it's currently on it should work the same version as long as it has internet access and doesn't force you to update.