The Total Payback update will add an additional six characters to the Mercenaries 2 mix, give players the ability to enter cheats and (this one's a biggie) enable cross-region support of Xbox Live multiplayer. Huzzah! Again, this is an update, meaning it'll be applied instantly and for free come October 13th.
Also, Pandemic revealed that they'll be releasing the first set of Mercs 2 downloadable content sometime this November. So, keep an eye out for that.