Help... Please.

PHP has started doing something strange.
After I upgrade from Apache 1.3.26 -> 1.3.27
and PHP 4.2.2 -> 4.2.3
I have started experiencing the follow behavior.
If I reference http://www.site.com/index.php
The page behaves normally.
If I reference http://www.site.com/
I get a download dialog box... that downloads the php source.
Has anyone seen this before... Know how to make it go away?

Thanks,

Jason