well, you need rotation to create a dynamo, and these rotate rather slowly; once per orbit only. Now, it does seem this is still quite fast enough to produce a dynamo, but still the issue is whether it can still be strong enough to create a sufficient magnetosphere to save the atmosphere from completely eroding away, given both how close and how active Proxima Cent. is .
Here's a paper that tries to see how much atmosphere planets in habitable zones around red dwarfs could lose given the activity of the star and the magnetosphere they could produce: - http://arxiv.org/abs/1204.0275 . I guess its quite possible various other models have different takes on it, but anyhow that's the worry. I hope we'll see such modelling work done for Proxima Cent. b soon.
