The Q4 earnings release included the 2012 fleet plan. Showed a net mainline decrease of 5 757s (offsetting 787s for 767 retirements) along with a decrease of 11 RJs. So shrinkage is already programmed into the numbers. You'll find the latest fleet plan here: United Continental Holdings, Inc. - Investor Relations - Fleet Information

They would need to retire ~2 mainline aircraft/month to offset combined LUAL/LCAL retirements. And when you consider that they'll need to fill a training pipeline, the number of aircraft retirements needs to be a bit higher at the leading edge of retirements.