See below from this article in the Sony section:
http://en.wikipedia.org/wiki/Regional_lockout
Additionally, some games separate online players per region, such as Metal Gear Solid 4 Online. PlayStation Store only contains content for its own country, for example the EU store will not supply usable map packs for an imported US copy of Call of Duty 4. In addition,downloadable content for the PlayStation 3 systems is region locked, so you need to buy DLC from US PlayStation Store to use it in a US game. More specifically, the PS3's file system includes region-of-origin, so DLC cannot be shared between different region games much like save files cannot.
