can i use my iphone 4 in india brought from us(at&t) and unlocked by at&t

Hi I unlocked my iphone from AT&T after my contract is over. i Planned to buy a new iphone going to get released.

My question is can i give this AT&T unlocked iphone to my dad in INDIA for his use. R is there any country restriction that even i unlocked my phone i have to use only in USA (Any Country lock will be there).

Let me know, Can i Upgrade my iphone to ios6 after release without problem in unlocked phone.

Thanks