Yes, you are not dreaming.
This is not an illusion.
It's Britney and Brittany.
Wearing the clothes from the music video of Me Against The Music.
Looks like Brittany' singing debut could be on this song!
Here's proof of director Ryan Murphy, Britney Spears and Heather Morris who plays Brittany on the show.
Furthermore, judging from the pic below by Hollywoodlife.com. There is a possibility that Baby One More Time is in too. Look at Lea Michele's outfit.
With each piece of news on Glee, either on the song, or some spoiler bits on the new season...it's definitely hard to sit still for the show. I'm GLEE-TASTIC!