The first day, I thought I had just forgotten to recharge the phone. But last night I saw that the battery was fully charged (green light) before I went to bed. This morning, I pick up the phone and it's telling me the battery is critically low! It was sitting idle the entire night, plugged into my computer (which was asleep).
This is my usual habit, to keep the phone plugged into the computer all night long, and I never had an issue until after the Mango update. Grrrrr.
My wife's phone doesn't appear to be suffering the same issue, so perhaps it's one of the apps I have installed that isn't playing nice with Mango. I'll have to start experimenting.