Nope, tried "F" and still did it. While reading my O'Reilly PHP book last night (I'm a beginner), they said printf is a rarely used function and they more or less said to use number_format() which I never even heard of.
Hhhmm, that seems odd. printf() might be a lesser used function, but its sister function sprintf() [uk2.php.net] - which uses the exact same syntax and AFAIK behaves the same as printf(), but returns a string rather than outputting it directly - certainly is!
printf() / sprintf() certainly come into their own when formatting a string of multiple values or of more complexity. number_format() [uk2.php.net] is also not locale aware in itself.
I'm curious, what version of PHP / OS have you tried this on? Have you tried another server?