This sounds like a job for CURL!
http://www.php.net/manual/en/ref.curl.php
It allows you to set your user agent, referring url, and then grab the file:
$ch = curl_init("http://target.site.com");
curl_setopt($ch,CURLOPT_USERAGENT,"Sammy Search Bot");
curl_setopt($ch,CURLOPT_REFERER,"some URL");
curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
$result=curl_exec ($ch);
curl_close ($ch);
If you're only grabbing html, you should be fine .. they can reverse lookup your IP, but by then it will be TOO LATE!! HAHAHAHAHA!