The only solution I found is not using wifi. If I have to use wifi, I reboot the phone after I disable it.
There's no wifi driver in official beta. This driver is not even a beta one and has not been validated by Sony.
There is no battery drain in ICS 4.0.3 either (unlike some says).
Currently, my battery last between 4 and 5 days without data activated (just a few calls and sms). It lasts 2.5 days with moderate data usage (an average of 15 hours with data and sync activated).