I bought Castlevania through this method using my credit card. I wouldn't "recommend" anyone does this in case the Valve machine decides to lock your account. It seems that the locked account punishment means you cannot make further financial transactions with that account, access Steam community features, Steam Wallet contents, and inventory items. You will still have access to your activated games.
Here's the bit in the Steam EULA that might get you concerned:
You agree that you will not use IP proxying or other methods to disguise the place of your residence, whether to circumvent geographical restrictions on game content, to purchase at pricing not applicable to your geography, or for any other purpose. If you do this, we may terminate your access to your Account.
I don't believe I am guilty of disguising the place of my residence (of course trying to convince the Valve corporate machine might be difficult).
When I bought Castlevania, the store throws up a message saying that my credit card country details do not match the store region, and it will re-calculate the correct prices (which didn't change because we pay in USD anyway).
After that, the transaction went through.
I did not use a VPN/proxy to obfuscate my IP address. The simple URL string change would be easily detectable as my IP address would show I'm connecting to the US webstore from South Africa.
According to web hearsay, Valve also validates your listed billing address against your IP address. This makes sense because they have to charge applicable taxes (and I don't think this bothers them in SA because they don't have a legal entity in this country - hence why we are still paying USD).
I looked around for info from others experiencing problems because of bypassing region restrictions, and it seems to centre around a lot of people buying lots of games cheaply from the Russian/Euro stores and gifting them to themselves and friends. Valve also doesn't like people doing a large amount of CC and PayPal chargebacks on their accounts, as this was apparently part of some scam in action. Apparently PayPal was being used a middle-man payment system to get games from cheaper regions, especially once Valve had flagged and blocked CCs being used for the abuse. All this activity was also taking place using VPN and not a simple URL string change.
As I mentioned, Valve also identified that my CC wasn't from the US when I paid, but let the transaction go through anyway. Obviously people do travel and access the web store while abroad, and they did have a system to check this since it flagged my actual residence location and recalculated the prices accordingly.
Maybe I took a risk here - guess I'll wait and see...
For a point of comparison, it would be interesting to try and buy a game through the UK store and see how the pricing is recalculated. Will we still pay in USD or GBP? Or, if a UK buyer tried to access the US store and if Valve kicks them back to UK pricing based on detecting their UK CC and residence details.
I hypothesise they are more concerned about people screwing them out of money than they are about "unrated" games being sold. I have activated many retail keys distributed for review purposes; for games not yet available in SA, or games not available on our store due to ratings. In general, it also seems that receiving game gifts and activating keys of region locked games goes down perfectly well. This all feeds my hypothesis that Valve doesn't really care unless you are actively and persistently scamming them out of money somehow.
I'm in contact with the FPB to try find out if they are behind the Castlevania game not being available. This would be similar to the Mark of the Ninja fiasco from a while back. Some of you may also recall that I was handing out Mark of the Ninja retail keys in a competition although the game wasn't rated for the SA store.
Strangely, I have noticed that sometimes these mysteriously absent games are available to us when they first launch. Castlevania was available for purchase when it launched on Steam (I know because I have been interested in the game since it was announced for PC). I was therefore surprised to see it unavailable when the special came around. IIRC this also happened with LA Noire and Mark of the Ninja.
Anyway, if my Steam account gets locked, you guys will be the first to know.