yep, they were the Feist video for Halloween (CRED)
Feist video costume

I forgot the SNL appearance was happening...

and this girl was Feist, as seen IN the video (CRED)
Feist costume

Feist - 1234 on SNL - Nov 3, 2007

& this is THE ACTUAL Feist, Halloween 2005 (CRED)
Feist

Feist - I Feel It All on SNL - Nov 3, 2007

Feist is huge. Amy Winehouse is huger.

UPDATE: one more late addition to the party:

Feist video costume