If they're just announcing it, why not?
They already have a non-IS version at just under $1300, why not add IS?
Is it a big seller? well, if the number of reviews at B&H are anything to go by, for every five 100-400's, they sell one 400/5.6L.
The announcements last year were 300/2.8L IS USM, 400/2.8L IS USM, 500/4L IS II USM, 600/4L IS II USM.
Notice that all four lenses have IS.
The *ONLY* white zoom lens that does not have IS is the EF-400/5.6L USM.
If it didn't sell, Canon would have axed it. Canon hasn't. Therefore I think it is ripe for updating.