Disagreed with JPS DHS RBG SGB Avg.
WHR 43% 37% 35% 30% 36.25%
CT 38% 35% 36% 32% 35.25%
AS 36% 31% 31% 28% 31.50%
AMK 31% 26% 24% 19% 25.00%
SOC 28% 24% 27% 18% 24.25%
So if WHR is replaced by a conservative justice, the right only maintains its position. It really has nothing to gain. However, if a AMK or SOC like justice is nominated and confirmed, then the left can gain some serious ground. The right knows this and will likely nominate a conservative justice to replace WHR. This means that the only real significant change we will see with WHR's replacement (if there is one) is that each term one more lucky law student will be a SCOTUS clerk (WHR only hires 3 clerks per term instead of 4 like every other justice).
Update: Even though WHR did not announce his retirement at today's session, he may still retire today as Lyle Denniston points out here in the more traditional off-the-bench press announcement.