TheMadScientist - 10:00 am on Feb 21, 2011 (gmt 0)
You might try marking one up as lang="en-uk" an the other using lang="en-us" with proper link relationships in the <head>
Designates substitute versions for the document in which the link occurs. When used together with the lang attribute, it implies a translated version of the document. When used together with the media attribute, it implies a version designed for a different medium (or media).
<link rel="Alternate" href="http://example.co.uk/" lang="en-uk"> on the us site; reverse it on the other.