It's been more than four months since I officially ran out of deodorant and started using baking soda on my underarms to prevent odors, and all I can say is that it really works.
In fact, I've been doing aerobics classes four times a week and even after my hour-long workouts, my armpits don't really smell. I will admit that the texture of the baking soda does create a bit of friction during my workouts, but the irritation is not bad enough to motivate me to stop me from using baking soda.
Summary: baking soda deodorant is still amazing!
Update - 3.22.09 - At the suggestion of some readers, I am now using baking soda AND cornstarch mixed together (1:1) and I am no longer experiencing any irritation. Thanks everyone!