I am on the process of creating an administrative boundaries layer based on OpenStreetMap data. Ideally, it should contain the different OSM administrative levels and its hierarchy, it should include existing administrative codes (ISO, FIPS, NUTS in Europe, etc) for each level and it should be updated regularly.
I am aware this is not a simple task, so I think on it as a mid-term project, but I hope to have some preliminary results soon (which I will publish here as soon as available) and then keep improving this basis little by little. I think such a layer would be very useful for a number of scenarios, for instance for scientific analysis involving administrative boundaries, specially in areas where such data is not publicly available. Moreover, it would probably attract collaborators to complete or correct the OSM boundaries where needed.
I have started analysing administrative boundaries tagging in OSM, tools to export and process the data, potential problems, etc. I have also posted the idea on [OSM-talk] mailing list and I have got lots of advises and proposals.
Regarding tagging, the current situation is a quite heterogeneous, as some country boundaries are mapped on the coastline, while other boundaries are mapped on the territorial water limit. Some countries try to map both of them, using a combination of tags which varies from country to country.
Regarding the hierarchy (for instance, which provinces are a subarea of a country), it is not usually specified on tagging, but it can be guessed (to some extent) from their geometric properties.
Existing third-party administrative codes (ISO, etc) are not widely tagged on OSM data, so they should be added some way, probably by creating a correspondence table that could also be used to ensure that there is no missing countries (or regions) in the export.
I will go deeper on this ideas in the following weeks, so keep reading!