Owners of Borderlands 2 post launch can still snap her up but at a price of $9.99 or 800 Microsoft Points. Gaige the Mechromancer releases simultaneously because she don't play favourites.
Gaige comes with a hulking great mech thingy which is apparently great for buying her time to dish some damage. She'll also compliment other characters with her skill tree.
Borderlands 2 releases on Xbox 360, PS3 and PC September 18th in the US, 21st in EU.