You need at least ROM3.0 to run OS3.1 but it can be a softkicked ROM also like in my A3000 with ROM1.4 that sofkicks 3.1. :-D
I belive however that you won't get all out of WB3.1 with only ROM3.0 and there's suppose to be some major bugfixes with the ROM3.1 aswell, also if you plan for further OS uppgrades ROM3.1 is a must.
