Our store is generally for the US. However, there are some products we will ship internationally. If an international visitor comes to our store they are encouraged to enter through the appropriate portal (Canadian shoppers enter here, for example). But there is no guarantee they will do so. So they can add a whole bunch of items to their cart only to find that most are not available to them. We would like to be able to say this item is available to this region (US by default) plus others, for example. If a customer enters through the appropriate portal, only those products available to them would be displayed. If they don't enter through the portal, when they go to check out items that are not available to them should be automatically removed from their cart with a message explaining why. Perhaps having a little indicator on the product display page also (indicating which regions this item is available to might be nice also).
related is: viewtopic.php?f=45&t=8494