Google Maps in WebKit: http://dl.dropbox.com/u/2128410/maps-webkit.png
Google Maps in Firefox: http://dl.dropbox.com/u/2128410/maps-firefox.png
Gmail in WebKit: http://dl.dropbox.com/u/2128410/gmail-webkit.png
Gmail in Firefox spoofing a WebKit UA header: http://dl.dropbox.com/u/2128410/gmail-firefox-webkit-ua.png
Gmail in Opera or Firefox normally: http://dl.dropbox.com/u/2128410/gmail-opera.png
Twitter in WebKit: http://dl.dropbox.com/u/2128410/twitter-webkit.png
Twitter in Firefox: http://dl.dropbox.com/u/2128410/twitter-firefox.png
These are just some of the big-name sites that I knew off-hand. At least these high-budget sites usually have some sort of fallback. Some sites just have degraded styles; some have parts that are unreadable. Some provide partial functionality to non-WebKit browsers; others won't work at all (like Bing web search in Opera).
This is becoming more common on the desktop too, with sites like Tweetdeck that just lock out non-WebKit browsers using UA sniffing: https://web.tweetdeck.com/web/unsupported.html