What is the most important factor in determining whether your site really counts when it comes to geo targeting a particular segment? With local search growing in prominence, there are several factors that help determine the relevance of local sites. But the most important may perhaps be whether or not your country specific top level domain matches the targeted geography.

An excerpt from SearchEngineJournal

A country specific top level domain is what really helps. Citing another Googler:

If a site is being hosted with a country-code top level domain, we generally take that as a really strong signal that this is the country the site is targeting.

So if you have a .co.uk top level domain and set geo targeting to the US, it won't help. It might help (though no one can be sure), if you own generic TLDs (.com or .net) which are not tied to any country.

It does make logical sense for a ccTLD to target users from that area, but should that be the all important factor? Perhaps with only a few engines dominating the search space there is little to argue about but just heed the rules.