Ok, I changed this once in 1.3.2, but now I realize the way I did it didn't really work either. The result is that if you're using PHP on a Windows server, it's not including the PEAR folder correctly. I think that the way I've got it now will work just fine for you. If you're using anything but Windows and not having a problem with this, you won't need to touch this code.
Fix:
Change line 24 of phpFlickr.php from:
if (strpos($_SERVER['SERVER_SOFTWARE'], "Windows") !== false) {
To:
if (strpos(__FILE__, ":") !== false) {