for the record, here is the correct way to mask a ubb url masked within a frame:
<html>
<head>
<title>
Your Forum Name
</title>
</head>
<frameset rows="100%,*" frameborder="0">
<frame src="http://yourubbaddress">
<frame src="whatever.html">
</frameset>
</html>
You'll find that the code in the post before this one does not work with Netscape as Netscape does not allow one frame within a frameset. The frameborder="0" is there for the sake of Opera browsers. "whatever.html" doesn't HAVE to be an actual page (as it doesn't show up), but you might want to make it a real page (blank or... whatever) to avoid all those errors it will generate every time your forum loads.
I'm adding this so late cause I was having a similar problem, and nothing came up in the searches; so this is my little contribution to your archive!
(ttol is a spammer! hehe)