She'll play Kitty Koslowski, a henchwoman working for Luthor.
Parker Posey, in her second super-hero film role (her first was Blade Trinity), will play Kitty Koslowski in Superman Returns, directed by Bryan Singer, according to the Hollywood Reporter.
Koslowski is a "villainous henchwoman" working for Lex Luthor, played by Kevin Spacey.
The rest of the cast includes Brandon Routh as Clark Kent/Superman, Kate Bosworth as 
Lois Lane, Hugh Laurie as Perry White, James Marsden as Richard White, Sam Huntington as Jimmy Olsen and Kal Penn as Stanford, another Luthor flunkie.
In related news, you can see Singer on the Tamworth set of the Kent Farm by clicking here.