Thanks all. The proximity sensor is working fine. But it is the way I'm holding the phone (coz it's so small!
my thumb sometime reach the button when I move the phone away from my face (to use dialpad, or turn on speakerphone, or....). It's annoying.
Now I'm trying some call replacement apps (Ultimate Call Screen HD Free). It's working but it does not look as "refined" as sony one. Wonder if I can replace with "stock" gingerbread dialer....
I didn't root my phone yet. It came with .62 firmware and I'm still don't have time to downgrade and root. I guess if ICS comes out, it will probably change the call interface as well.