All the bars I went into in Arinsal had wireless and would provide the WEP key for you when you asked. Derby's, El Cau, Michlou and Bogarts all had wirelss.
Cheers for the great feedback, and your booking.
Quo Vadis, the bar downstairs, has wifi - just ask at the bar for the password. I think you can get it in the apartments too but not 100% sure.