Players who pre-order Star Wars: The Old Republic, and redeem their pre-order codes, will gain early access to download and play the game before the release date. Senior Online Community Manager Stephen Reid explains that it will be on a first-come, first-served basis, based on the redemption date of players' pre-order codes. An exact date has not been announced, but, they want to allow for game testiing, pre-orders, and gradual population ramp up to ensure a smooth launch day. You can find out more in the Pre-order FAQ and early access section.
(Source: SWTOR Forums)