Lauren Fausts Deviant Art page asked her about the lack of Rarity in the recent episode. Apparently there was an entire planned sequence with her that got cut. Poor girl, its a shame FiM isn't a 45 minute long show each week instead!
And the responses for the image hating people out there:
Lovemebuthateme: loved episode 4 <3, may I ask why rarity wasn't in it?
Lauren: She was in the script. Sounds like she got cut.
Kitmit: what was she suppose to do in the script? You think she would of made herself some fabulous nightmare night costume for herself.
Lauren: She was still working on her costume, and perfectionist as she is, it wasn't ready. (I forget what it was.)
There was a whole sequence where Twilight brought Luna to Rarity's for a makeover, hoping she could give Luna a "softer," less intimidating look. Rarity freaked out about getting to dress up a Princess and over did it, covering her in the pinkiest pink princess get up ever. Luna disliked it and in taking it off, had a piece of the dress in her mouth. Pinkie showed up just at that moment and screamed that Nightmare Moon ate a princess and all the kids screamed and ran.
Thanks to Kitmit for the heads up!