Mobile authenticator are only free to the Apple products like iPhone and iTouch (not sure about iPad).

For other hand phones, you can check for the compatibility at http://mobile.blizzard.com/us-en/support-compat.html/ . X means that phone model can support that feature.

You do need either a PayPal acc or credit card to buy it and it only cost 1 USD.

I just bought my mobile authenticator today for my n95 and it is working fine so far. I am going to attached it to my account when I head back home. (I don't trust this office computer =P )