abolat: This pretty much. I wonder, however, what would happen if someone really had a home in another region as well as a network and set up a VPN to his own home... would that still count as being against the terms of service, considering that the person has logged on from that IP numerous times when he was at home?
As I said, they will threaten to disable your account if they become aware of that. Case in point: Someone from the US temp. moved to Japan and remote connected to their PC in the US to purchase Dungeon Siege 3 (hoping to get around the delayed Japan release). They managed to purchase the game, but obviously were still stuck the Japanese release date. When the contacted support about the release date, support replied that they had every right to disable his account for the way he purchased the game (but less politely).
Of course, if he hadn't told them, they probably would never have known.
In order to make purchases in different region to the one tied to your Steam account, you generally need a billing method in the other region. Most people using VPNs don't have that and generally provide a false address (and then hope that their normal billing method works -- some won't).
Valve tend to become aware of this fairly quickly (perhaps via a list of known VPNs, etc., and disable any account caught.
If you tend to move countries a lot, then tell support and they will generally set your account so that you can use the Steam store in your "home" region via a browser using ?cc=XX (but not the client). But you must prove to them that you have valid billing information for that region.
That's one reason I regret not keeping a UK credit card...