Unless you're stuck with using a earlier version of PHP3, strip_tags() will do everything you want! According to the manual, "Allowable_tags [the 2nd parameter] was added in PHP 3.0.13, PHP4B3.".
I use this all the time; it works great, or rather, it did once I figured out how to include multiple tags--the manual explained that you could do it, but gave neither explanation nor example of how to do it. The key is to separate them by commas.
So, for example, if you're going to be very hard-nosed:
$text = strip_tags( $text, '<b>,<i>');