In pretty standard Samsung tradition, Samsung has released kernel source for a device that hasn’t yet been released. The newest kernel drop is for three variants of the Galaxy S 4, namely the GT-I9500 (international 3G model with the Exynos octa core), The GT-I9505 (international LTE model with the Snapdragon 600), and the SGH-I337M (the Virgin Mobile Canada model). And looking at past patterns, we can assume that the I337M source code will nearly match that for the AT&T model, which logically should the the SGH-I337.
If you want to start modding, hit the source links to download the source. Have some fun with it, and hopefully we’ll see the developer community for this device explode.