This was discussed in a recent thread (
http://gbatemp.net/threads/possible-to-hack-amiibo.373149)
Short answer: no. NFC isn't a simple wireless barcode. The best way to imagine the amiibo is as a very weak, low-powered computer. When it comes close to an NFC reader, the reader transmits enough electricity to it, wirelessly, to turn it on. Then, they start talking to each other. Since they are both computers, the information can be encrypted (very well). The only way to decrypt it, is to know the keys.
This isn't entirely accurate, but it's close enough that you get an idea of how hard it will be to hack, and also why you shouldn't expect to just scan it with your phone, then use your phone instead of an amiibo. Also I'm not sure if they use commonly accepted standards, or have any propriety ones. If they have propriety ones, then in all likelihood your phone wouldn't be able to emulate them, without a never-to-come firmware upgrade to your phone's NFC driver. To put this simply - the NFC language Nintendo uses, might not be the commonly-accepted language. It's like your phone speaks the international language of English, but the amiibo is programmed to only speak Klingon - and this is at the hardware-firmware level, so you can't download an app to translate for you.
If someone feels I'm wrong about anything, please feel free to correct me.