Dean Rowe, the SEO manager at Burberry's headquarters, received one of those Big traffic change for top URL notifications in Google Webmaster Tools for the http://us.burberry.com URL.
In fact, Google has currently delisted the us.burberry.com page. Why? Well, it was not a penalty, it was because Google felt the page was redundant to the uk.burberry.com web page.
Google's John Mueller told Dean, "We're currently seeing the UK and the US versions as being identical, and picking one of them (in this case, the UK version) for indexing."
A great way to resolve this is to make sure that they're really unique. Another solution could be to use geotargeting in Webmaster Tools (if you aren't using it already), and to use the "link rel=alternate hreflang" construct to make sure that we understand the relationship between your UK and US pages. You can find out more about this at http://support.google.com/webmasters/bin/answer.py?hl=en&answer=189077
Here is a screen shot, which might change in the near future:
Burberry is a huge brand and looking at how Google handled this situation is something most SEOs can learn from.
Forum discussion at Google Webmaster Help.