I saw a thread on Gaintbomb websites about “Can i use a vpn to unlock Far Cry 3 Earlier?”.A lot of gamers are discussing about this. There's debate concerning the risk of losing your account over this. I can tell you I used a VPN to unlock Deus Ex last year and I had no problem (although I kept steam offline untill the european release).
My understanding is that Valve will come after you if you use a VPN to purchase games at a lower price in another region. Unlocking games seems risk free from what I understand.