I do this and variations all the time. It is standard practice. I presume you are worried about duplicate content penalties? I wouldn't worry so much, just make sure you are not loading the page down with too much unnecessaryness :O) Look at this site, webmasterworld, and how well it does in the serps. Standard template and no penalties ..
There's no problem with this - but make sure that your <title> is different for each page, as well as the meta description and keywords if you use them. You can use a variable at the top of each page to echo the values you want in the right place, for example: <?php $pagetitle="page title"; $keywords="list of meta keywords"; $description="your meta description here"; include($_SERVER["DOCUMENT_ROOT"]."/includes/header.inc.php");?>