Expat taxes and residency
While I'm not a lawyer, I've lived in the UK for 14 years now and have a good practical knowledge of what's involved in living overseas and filing taxes.
Domicile means "the place where you normally live" in this context. So, my domicile changed from California to the United Kingdom when I moved here in January 1994.
For voting purposes, I vote only in Federal elections in California (including primary elections). I do not vote in state elections as this might convey to the state that I'm planning to return. For the poster who wanted to know where her child, born in Japan but a US citizen by descent, would vote: s/he should vote in the same place that you last were registered in the United States.
If you vote in state elections the state can infer that you want to come back at some point in the future. And, if you do return to the same state after a period abroad, they will likely hit you for as much back tax as they legally can along with penalties. So, if you return to the US, live in another state for a while to obviate the chance of getting soaked for back taxes.
I am unsure about the effect of keeping a driver's license in the state in which you were living when you moved overseas, as I don't drive now and my only license (from Illinois) expired 10 years ago.
The current expat tax laws have changed yet again, and not in a good way. In 2003 Congress almost removed the foreign earned income exclusion--the Senate came within one vote of doing it. It is still there, but if you live in a low tax country (Singapore, for example) you will get socked for income above the exclusion amount. Some self-employed people may get hit with the Alternative Minimum Tax, although the rules for that have changed yet again this year, I understand.
If you can, best go to a US tax accountant practicing in your country to have him or her make out your tax return. I have never done this before, but I'm going to do it this year. As the UK's tax rate is higher than the US one, I am not likely to have to pay much if anything. However, I'd rather a professional told me that than try to work it out myself.
I am also a UK citizen, and am considering renouncing my US citizenship as I don't intend to move back to the US and find the red tape of having to fill out a tax return each year a bore. There are tax implications to this as well, but I don't think that in my case (as I don't own property or have a massive amount of money) they will apply. I'm going to talk to the accountant about it when he does my taxes, though.