A common but difficult question was asked at HighRanking Forums about the issue of targeting different countries.

My advice for when it comes to targeting different countries is almost always to have two different sites on TLDs (top level domains) that match that country. So if you want to target the US and the UK, have one US site on the .com and one UK site on the co.uk and make sure your content is localized for that market. It might help to also host each site in that region, plus it is always good to set the geographic target for the UK site in Google Webmaster Tools (I would leave the US version untargeted in webmaster tools).

High Rankings administrator, Randy, adds:

Forum discussion at HighRanking Forums.