WebApp Sec: Preventing cross site scripting

[Andrew Beverley <andy () andybev com>]

I am currently writing a web application that, as a small part of it, 
needs to display an email message. Obviously the message is potentially 
in html format, which to display could be sent straight to the browser.
I would like to know the best way of filtering out undesirable html. I 
understand the best way is to only allow acceptable information, in this 
case all the different html formatting tags.
However, there is a lot of tags that are acceptable. Another approach 
would be to strip out all the bad stuff such as <SCRIPT>, <OBJECT>, 
<APPLET>, and <EMBED> but this is far from ideal because of new tags 
becoming available and so on.
Are there any functions available (for php) that will take a html page 
as input and strip out all nasty stuff? Does anyone have suggestions as 
to how to do this as easy as possible?
Thanks,

Andrew Beverley