It's not very often that an observation about articulatory phonetics goes viral. Josef Fruehwald points out a rare example ("Britney Spears tongue", 8/19/2010):
This is apparently not just a (tongue-and-)lip syncing quirk — Joe notes a live performance where she does the same thing:
As Joe observes, there are no apico-labial /l/'s visible in Ms. Spears' interviews, so this is purely a singing (or pretending-to-sing) feature.
John Wells discussed this yesterday ("Linguolabials"), noting that
Linguolabials are found in the consonant inventories of very few languages. Ladefoged and Maddieson, in their book The Sounds of the World’s Languages (Blackwell, 1996), give some examples from Tangoa, a language spoken on an island belonging to Vanuatu, in which linguolabials contrast with both bilabials and alveolars for plosives, nasals and fricatives …
John also points out that Ms. Spears gives visible tongue in singing /θ/ and /ð/ as well as /l/, and speculates that "she thinks it’s sexy". But if that's the explanation, why doesn't she do it at least sometimes in interviews?