A Google Webmaster Help thread discusses a unique issue to a few niche sites that have no choice but to block all users from the United States while also wanting to allow GoogleBot to access the site.
Since GoogleBot lives in the US, blocking US based IP addresses, would likely also block GoogleBot. That would result in Google not indexing your site and you not ranking well in Google for the country you are targeting.
This solution isn't necessarily new, but it is the first time I have seen a Googler suggest it in a forum.
Forum discussion at Google Webmaster Help.