Well, Zend have an optimiser that does something like that, and a search ought to turn up various PHP caching packages and/or articles. I usually wouldn't worry, though, unless it's a big include and a very busy site. PHP runs its own compiler over the code at load time.
(One thing you might like to consider doing yourself is a maintenance utility that you can run when you add pages; it looks through the PHP source file, finds references to include('busyinclude.php'); and replaces them with the contents of busyinclude.php - in other words, do the include yourself in advance. Keep the originals stored somewhere in case you want to do any editing.
I started writing something like this myself once. It has since turned into a an entire preprocessing site deployment package and a hairball in serious need of a restyling!)
