Add support for the <pre> tag.  This would allow content to be added to the HtmlTextBox without having to worry about encoding it properly.  We use the HtmlTextBox frequently in situations where we want to apply mixed formatting.  

This would help in several scenarios.  One scenario that came up recently was when I wanted to render literal space.  I got around this by replacing every other space with a non-breaking space but the pre tag could have made this much easier.  Other situations involve getting text from the database that could include a < or > symbol which we have also got around making sure we run the text through an HtmlEncode routine.