Going by not entirely public NPD sales figures (in North America), Gearbox has determined that Borderlands had the most launch-week sales of any new IP in 2009. The only other title that could contest that would be Prototype, which enjoyed greater sales on the Xbox 360 platform (but just by 1,900 units) than did Borderlands when comparing the two games' respective launch months (419.9k vs. 418k units). However, Prototype, which released June 9, was available at retail for 11 more days during its launch month than was Borderlands (October 20). While we don't know total sales of either title during their launch months (both were released on Xbox 360, PS3 and PC), it's reasonable to assume that Borderlands is the fast-selling new IP (during a launch week), if not the best-selling one in combined platform sales for a launch month in 2009.
So, congratulations! But watch out for Dragon Age: Origins -- we hear it's fast, too.