Do the calibre server logs show anything? Preferences | Sharing over the net | View server logs?
Mine show:
192.168.2.189 - - [06/Jun/2013:18:59:29] "GET / HTTP/1.1" 200 15487 "" "Mozilla/5.0 (Linux; U; Android 2.0; en-us

AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1 (Kobo Touch)"
192.168.2.189 - - [06/Jun/2013:18:59:29] "GET /mobile/style.css HTTP/1.1" 200 1758 "http://192.168.2.177:8080/" "Mozilla/5.0 (Linux; U; Android 2.0; en-us

AppleWebKit/533.1 (KHTML, like Gecko) Version/4.0 Mobile Safari/533.1 (Kobo Touch)"