I've had my Atrix since March, and I've noticed no change in the Battery life from 2.2 to 2.3. I've ALWAYS had to charge my phone multiple times a day. I attribute this to the dual core CPU, but MAINLY due to poor signal strength in my area.
For example, I have to charge 2 to 3 times a day with moderate use. My old BB 9700 that other people reported 2 to 3 days on a charge would barely get 1 day.
My fiance lives in Orlando where they have VERY strong signal. Her Atrix lasts almost 2 days. When I'm down there, I get almost 2 days on mine. (If it was a problem with the Phone, or the Update, then that would not change just because of where I am. - The Lowest Common Denominator here is the network where I am.)
I propose that the problem you have is not related to the GB update to 2.3, but rather a bad tower in your area, or AT&T working on the towers causing problems with the signal. I would contact them about it.
Do you notice your phone dropping to E (Edge/2G) more often than before or Fewer bars of signal?
When the signal quality drops, the phone has to boost power to the radio's to stay in contact. This means a faster battery drain.
The other thing to check is that all of your apps are updated through the market. Check each of their pages for other people reporting high battery drain on that app. It could be a bad release of an app (Every developer does that from time to time..). Or, an app that is not fully GB compatible.