<p>This happened to me in Visual Studio 2008 pre and post 2008 sp1 on more than one computer and to someone else I know, so it can't be an isolated incident.</p>

<p>Seemingly random, every so often I lose all syntax highlighting in my aspx page (the html) so that Visual Studio now looks like a really expensive version of notepad.</p>

<p>Does anyone know why does happens?  Better yet, anyone know how to fix it?</p>

You have basically experienced Visual Studio giving up.  It gives up for many reason, the page is too complicated to highlight which is usually because there are too many syntax errors.  The highlighting is done with some very complicated and intelligent RegEx statements essentially, however if Visual Studio is unable to apply them it just displays black text.
One thing to try is to fix all the syntax issues, if any.
By the way this "giving up" happens in most IDE's.  I have seen it happen once or twice in Elcipse too.