Some ISPs block port 80 access to home accounts to prevent commercial use. You might try another port.
Can you post the source of a sample web page, i.e. what the browser actually receives? I don't want to have to wade through the code guessing what it does. Most browsers provide a means to show the source of the currently loaded page. Just paste the source here, in CODE tags.
<center><p><h1>Welcome to the DBM Switch Controller </h1></p> <hr><br><form METHOD=get action="http://192.168.1.15/"><h2> REMOTE SWITCH: </h2> <h1><font color="#00FF00"> ON </font></h1><br> <input type=hidden name=cmd value=3><input type=submit value="Switch off"></form>
The only IP address in that page source is the local address of your Arduino, should it not be the public address of your router ?