I agree with Greg and Tom. Politically anything south of I80 is considered "Southern" Illinois. But for tag purposes I say we split the state closer to the middle of the state.
If you look at Ill route 136 it is just south of Bloomington and runs pretty much east and west. Looks like a good place to divide the state to me.
I'd say I-74 as it's easier to recognize.
"Ride Illinois, 11 turns in 317 miles"
(also used for Florida and a couple other flat states.