It will be American Top Team vs. Nova Uniao on New Year's Day, according to MMA Junkie:

Former champion Mike Brown and fellow featherweight contender Diego Nunes now have a likely date for their post-WEC UFC debuts.

Sources close to the negotiations told MMAjunkie.com the fighters have agreed to meet at UFC 125, and the bout soon should be finalized.

Mike Brown (24-6)
Win Cole Province - TKO (punches) WEC 51
Loss Manny Gamburyan - KO (Punches) WEC 48
Win Anthony Morrison - Submission (Rear Naked Choke) WEC 46

Diego Nunes (15-1)
Win Tyler Toner - Decision (unanimous) WEC 51
Win Raphael Assuncao - Decision (unanimous) WEC 49
Loss L.C. Davis - Decision (Unanimous) WEC 44

Jan 1st, live on Pay-per-view from Las Vegas, Nevada:

Main card:
Frankie Edgar vs. Gray Maynard [LW title]
Jose Aldo vs. Josh Grispi [FW title]
Clay Guida vs. Takanori Gomi
Nate Diaz vs. Dong Hyun Kim
Chris Leben vs. Brian Stann

Preliminary card:
Brandon Vera vs. Thiago Silva
Jacob Volkmann vs. Antonio McKee
Mike Brown vs. Diego Nunes
Cole Miller vs. Matt Wiman
Phil Baroni vs. Brad Tavares