Dan Coulter spilled the beans on the developer list:

On Feb 12, 2008 1:53 AM, {removed} <{also}@{removed}.com> wrote:
    Can some one tell me , in which Progaming language has flickr been
    developed
PHP
    and can we get the clone
Highly unlikely
    how can we develop the same application line this in php.......
Hire a world class team and pay them a crapload of money.

The bastard :)