Details here. I believe a Steam pre-load is consistent with how Shogun II and Napoleon went down.
Also, apparently R2 will be contributing to the hat-based economy on Steam.
Pre-Purchase ROME II with Steam and not only get the Greek States DLC but also the Greek Hellenic Armor for The Pyro, the Egyptian Cheetah set for The Scout and now the Barbarian Set for the Demo in TF2! More to come! Also, we can confirm that pre-loading on Steam will be happening before release on Sept 3rd.