It seems like a lot of effort to go to when (IMO, as someone who has done a LOT of internet development) an AVR is a pretty weak CPU on which to write internet applications.
My internet development has all been on PCs running linux (working at the socket level), so I guess I'm not in the habbit of considering hardware requirments.
What I envision for the Arduino is pretty simple though. I am thinking of a device that will connect, either send a variable or receive one, and disconnect. So you just need to connect, send a command, possibly receive a reply depending on the command, and disconnect. That leaves most of the work on the server (which can be a full workstation). If you want to display a web page, you send the key data (temperature, voltage, etc) in the manner I described, the server logs it and generates the web pages on demand. Similarly to send information to the Arduino, the server can collect whatever data is needed, parse it, and give the result to the Arduino.
I don't think that should be too hard to implement, though I am not too familiar with the hardware and mac layers.
I doubt whether they'd pay for themselves.
They don't have to pay for themselves, it's a development expense, the Arduino team is getting free labour for the loss on the boards. But it shouldn't be that bad to make 10-20 if they don't have the fab assemble the boards (and remember the target of these boards is the more technical users who can do some basic surface mount work).