Assuming you have installed PostgreSQL and your WAMP installation is on c:\wamp, you will need to copy
c:\wamp\bin\php\php5.3.9\libpq.dll to c:\wamp\bin\apache\Apache2.2.11\bin
Make sure you also have the following filesC:\wamp\bin\php\php5.3.9\ext\php_pdo_pgsql.dll and
Also, make sure you have enabled the above 2 files as extensions, either via the WAMP menu (click on WAMP icon on taskbar, PHP, PHPExtensions, find the above 2 and 'check' them)
Please note that php5.3.9 and Apache2.2.11 refer to my specific PHP and Apache versions.
Adjust those to suit your installation.
ps: Comments and/or links to this article are most welcome!