Pity poor Christina Aguilera. Britney Spears gets all the attention, even when she "snogs" with Madonna.
The problem is that she's not applying basic corporate strategy principles. Spears is the leading brand in the "pop tart" category, and any other competitors have to recognize that and carve out their own niche.
Mandy Moore, for example, dyed her hair brown, and has received good reviews for her various movie roles. "Brunette thinking-man's pop star" is significantly differentiated from "blonde jailbait slut."
Aguilera, on the other hand, seems to have settled on the strategy of out-slutting Spears. This is a difficult strategy to pull off, as anyone who has tried to compete with Wal-Mart on price can attest.
Aguilera would be better off focusing on her core competencies (greater vocal range and ability to convey emotion) rather than competing on sheer harlotry.