PHP-5.2.3 and "Internal Server Error"

On one of our test boxen, we recently upgraded to PHP-5.2.3 and all of a sudden started getting Internal Server Errors. The Apache log file tells me that there is a premature end of script error on index.php, which is simply an entry point script to an MVC framework. I have checked that there is no whitespace after the closing ?> tag and all, and have removed almost everything that I can think of that may be causing something like this. It seems to choke on the call to ob_start()... Anyone have any ideas that may shed some light on this one? Appreciate any helpful tips. --Paul