I found this interesting piece in the NY Times about 2008 presidential primary donations from US citizens overseas. It looks like they will continue to play a large role in the upcoming election.
Driven largely by Mr. Giuliani and Mr. Obama, contributions from American supporters in other countries are on pace to far exceed the sums raised overseas in the 2004 election.
By the end of June, the candidates had raised a combined total of $551,000 abroad, compared with $31,525 at the same point in 2003 and nearly $1 million for the entire election cycle.
...
Mr. Obama, who has raised more money from abroad than all other candidates, $251,000, appears to benefit from a successful Internet operation. His Beijing videoconference appears to have been a success. He has collected nearly $25,000 from donors in China, the most of all candidates.
Mr. Giuliani had raised the second most from overseas, at $121,550, even before his fund-raiser this week. Most of that came from London finance executives, including $11,300 from executives of Elliott Advisers, the London affiliate of a company owned by Paul E. Singer, a hedge fund executive who has been a main supporter and adviser for Mr. Giuliani.
The piece also had a nice graphical breakdown of the money by country:
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As you can see, the top fundraisers on military bases abroad have been Obama, Edwards, and McCain. While McCain can be explained by his past military experience, Obama and Edwards look to be gaining their support from their strong calls for getting our troops out of Iraq.