This information is scattered in a couple comments in the PHP manual, but as far as I can tell, it is not well documented. This may change once PHP5 is released.
Problem: When setting up a Windows/PHP5/MySQL/Apache I have correctly pointed the php.ini file to php_mysql.dll, but on apache start I get the error message "cannot find <path>/php_mysql.dll"
Solution: Apache is incorrectly identifying the dll it cannot find. It is actually looking for the libmysql.dll located in either your PHP root, or your extensions folder. Placing this file in either your SYSTEM32 folder, or your Apache root will fix the problem.