It is not that the two singers are going to act in Eclipse, but rather that they may do the soundtrack.
It is not for sure yet, but cutie Justin Bieber, and Adam Lambert might do the soundtrack for Eclipse. Adam may contribute with song"Suburban Decay".
It is true that many of the movie's fans are also Justin fans, so it would be pretty cool if he helped with the soundtrack. The only thing is, would any of Justin's songs actually fit with Eclipse?
We're not so sure...