I'm thinking about registered a domain in the UK (.co.uk) of a product that is a trademark in the US, but not in the UK. The company name is a trademark in the UK. It's in the form <company><product>.co.uk Any reason not to? PS. I searched the UK Trademark's here: http://www.ipo.gov.uk/tm/t-find/t-find-text/tmtsearch-default.aspx
an issue is that if your site is accessible or does business in the u.s., then you will run into potential conflicts. it's always best to be safe than sorry or have the rug swept out from under you once you are having some success.
I also believe that US has treaties with other countries to protect federal marks internationally. If it's in the same class of product, it could be a problem. Consult some more research.
If the company name is trademarked in the UK and you are using it in your domain name, you'll probably run into trouble under UK trademark laws too.