So what has to be done to solve the problem? 1. Inform the geofabrik people that they may increase their boundaries. maybe they could include the three or twelf-mile-zone around a country?
2. The format of an OSM dump should be extended by the possibility to define the exact borders of the OSM dump. The osmosis tool could then add the boundary used to extract the OSM dump to the dump. This solves 99% of the problems originated by inexact dumps and unknown boundaries.
I have started a discussion on the osmosis mailing list: http://lists.openstreetmap.org/pipermail/osmosis-dev/2010-January/000452.htm... Hopefully the osmosis people support our requirements. Then a lot of discussions and "guess how to close an unclosed polygon algorithms" are superfluous. WanMil