This is really cool, great work!

The only problem I can see is that it doesn't work with sites that use user agent sniffing serve up mobile sites (see bbc.co.uk - http://dfcb.github.com/Responsivator/?site=bbc.co.uk) I realise that's not technically 'responsive', but it would be cool if responsivator could handle that as well.

Having said all that, I don't think it's even possible to spoof different user agents in iframes(?), maybe could be done using a headless browser to take screenshots?