Why exclude people from the US? they can still refer people who are not from the US.
My main suggestions to get more members from outside the US would be to globablize the forum, in other words make it less American.
A prime example of this is the sports section, one of the more popular threads at the moment is the "Euro 2008" thread, it sits within the 'other sports' section <<<the biggest and most popular sport in the world (football/soccer) is classed as a 'other sport', for a European that is just crazy.
Everything here is US orientated. For me I stumbled upon this forum after typing General Forum into Google, but most people find forums through their interests, an example; you have an interest in motor sports you find a motor sports forum, if they found this forum they would find only US motor sports, politics = US politics, Religion = US viewpoint, Sub-talk = US style banter, News = US current affairs etcetera etcetera.
Of course having a majority US membership the above arrives naturally and is unpreventable but as a non-US member a large portion of it is of disinterest, irrelevance or simply not understood (the last being a common US trait of assuming what is said is common knowledge and not realising there is a whole world out there who don't know what Mountain Dew is? what Arby's might consist of? who is John Letterman? or where or what MA, FL, CA stands for?).
If you want a global membership you have to try to appeal to a global audience. Not just make the forum a little more European and Global but to make them a little less American, as an outsider* it is a turn-off.
*to feel an outsider sums it up well, its the everywhere else but here attitude; it does not bode well.
Im not having a go at the forums or the North Americans. Just offering a recommendation of the most obvious influence/flaw from the alternate viewpoint of a newish member who is not from the US.
I hope I have explained myself well, it would defiantly be worth while if you(Hybrix) go browsing a few .co.uk or .au forums and just try getting the feel of them. There is a defiantly difference in personality and attitude throughout the forums of different nations than those of the US.