"Very, very interestingly, Steven has told Alex Kingston, who plays River, everything. Everything, about the whole story, about the whole arc, about where it’s all going, because her character is meant to know because she’s from the Doctor’s future. But told me nothing. So Alex knows. Which is infuriating." -Matt Smith. So when she says "Spoilers," she means it.
It was perfect, really. Bringing Rose back in the way we had expected would have taken much longer and would have detracted from the Doctor. This way told us so much about her and their relationship, even in a roundabout way, it was perfect.