Assuming no limit on toppings per pizza, it's actually 1,124,000,727,777,607,680,000. There's a limit of four toppings per pizza, so it's closer to 3.6 trillion possible combinations. Real close to six million, eh?
Vogon adds "She's a Kelso [from That 70's Show]".
And for the mathematically inclined, check out the monzy.com post titled "The Waffle House is bad at combinatorics."