We have one paladin tank in our raid lineup, and he uses it sporadically (I'm holy and I have Grid set up to show it so that I don't overwrite another one with mine). Haven't detected any rhyme or reason when he does use it, and it's possible that he only does when he notices that he hasn't gotten it from a healer. With the upcoming nerf (only one SS up per caster), it will be more important for every paladin to keep one up, even with multiple in the raid.
As a healer, I use it, um, religiously. No reason not to shield a paladin or DK, and SS isn't powerful enough to rage-starve a warrior or bear. Although this might change with 3.1, at least for Ret - but Ret would be shielding themselves, not the tank, and even with the improved version, Holy doesn't have enough SP to make it a true bubble.