Are you experiencing running issues? Overheating? Any other problems? Smoke can be a sign of an engine issue, but it may not be. Depends on what else you are experiencing, and the severity of what you are seeing.
Any time you overheat an engine - especially a small, light engine like the one in the Focus, you run the risk of head gasket or head damage. You need to do some testing to find out if you indeed have an issue with that.