I would not make this trade. Favre has a tendonitis issue that could keep him out of 1-2 games, then flare right back up and keep him out of 1-2 games again later. Contrary to popular belief, the Bears showed that they CAN compete on the pro level with their run game, and you should expect them to bring a much more balanced attack in future weeks. While this might mean fewer pass attempts for Cutler, it will actually take pressure off him and make him more productive. Bottom line, Cutler is a better fantasy QB than Favre right now, so you'd be giving up a starting RB to get a backup QB, which is not a good trade.
12 team H2H league, standard scoring, no PPR.
QB: Cutler, Kitna
RB: All Day AP, CJ2K, F.Jax, R.Brown, S. Greene, Dixon
WR: G. Jennings, Roy Williams, M. Williams (sea)
TE: J. Tamme
K: Hartley, Carpenter