One of my domains economicshelp.org gets75% of traffic from the UK. However, I had to switch from a UK host to a US host (because the UK host wasn’t giving good wordpress support)
I was concerned that this could lose the UK traffic. I did buy the .co.uk domain and mirrored that to the .org.
One tip I picked up is that in Google Webmasters tools, you can give a website a specific Country target. By default it is left at non country specific. For some websites it is very important to target a certain geographical niche. If you host on a foreign (US server) then selecting this option will enable you to rank in local searches. like Google.co.uk
Recently I have noticed that Google is giving much more weight to country specific searches. Search results can vary quite a lot from one country to another.
BTW: Netwriting.co.uk gets less than 50% of traffic from the UK, despite having a .co.uk domain name. But, the information here is not country specific I would leave the option at Google webmaster tools at non country specific.
I wrote previously about the benefits of having a .co.uk domain name
(It also helps that .co.uk domain names are cheap to pick up at less than $5. I know some countries like .com.au and .ie charge a very high amount.
There are some useful comments on this post at Problogger (in fact the comments are much more informative than the post itself) – the joy of having 46,000 subscribers – the readers will write the blog for you!