Hi Morag

I think you'll find that now that you're a virtual service provider you have a lot more flexibility in terms of who you work with - in other words, it doesn't matter...the entire four years I was abroad I was working with clients in the States and Canada and ALL of them *loved* the fact that I was literally working on their business growth while they were out at the movies, having dinner, or sleeping. Market yourself in such a way that your clients understand you literally extend the business day to 24 hours and you'll find clients in all geographic locations are very happy to work with you.
As to the physical address - I never provided one, even on my business site....it wasn't an issue. All of my clients have always known where we are (we keep them posted on our travels and numerous moves) and really do enjoy the 'story' of our travels not to mention the unique relationship with a truly global service provider.
Your location is part of your advantage - use it as such and you'll do just fine!
On a side note, now that we're in the States I find the responsiblity of managing my time in an equally beneficial manner (to my clients) falls solely on me. They're used to getting things 'tomorrow morning' but have all been great - they know where we are now and make their requests and note deadlines accordingly. When you're working in an opposite time zone you get really good at helping clients understand how much time is needed to get something done - you'll get used to it!