For what it's worth, according to MTV News, some Britney fan sites are debating the authenticity of the track, questioning whether it is in fact Spears' vocals. But it certainly sounds like it is. This demo version of "Telephone" features the same plucked strings, driving electro beat and digital-stutter of Gaga's released version.
With all the production effects added to the vocals (and with Internet pranks being what they are), we're putting the odds at this being a fake at 50/50. But you never know! It certainly sounds like her.
What do you think? Real? Fake? Let us know in the comments!