PHP 5.2.8 is refusing to load php_pgsql.dll, with the following error:
Warning: PHP Startup: Unable to load dynamic library \'D:\\PHP\\ext\\
Just a thought -- make sure that the Postgres binaries are in the SYSTEM path, not your user-specific path. As it happens, that was the problem on my machine. :-)
Edit:
Come to think of it, that explains why Dependency Walker would report all A-OK but the problem would still persist -- you run DW under your own account, while Apache runs as SYSTEM, and hence won't have your personal PATH settings available.