When Diana Kerry came to speak in Vancouver, her message was largely about Americans overseas voting. I've added a couple of links in the Politics section to your right to sites that will help Americans overseas get absentee ballots.
I can vouch for the fact that overseasvote2004 works, because they got me all the necessary forms to arrange for an absentee ballot here and I received my modified voter registration card from Snohomish county a few days ago. I don't know for sure about TellAnAmericanToVote but I imagine they are presenting essentially the same information.
It's not hard to do, but it varies by state and the entire package of information that might be necessary is a large book (in order to know the exact requirements and deadlines for each state), so having all the necessary information online is wonderful.
The short version is: if you are an American citizen living overseas and of age to vote (18 years) in a federal election, you are entitled to an absentee ballot. You register in the state where you last lived (or if you have never lived in America, where your parents last lived -- I'm not sure what you do if you've become an American citizen through marriage and never lived in the US, but I'm sure the web sites will inform you). Voting exclusively in a federal election (voting for President, Vice-President, Senators, and US Congresscritters) will NOT effect your tax position or current legal residence status.
There are approximately 5 million Americans of voting age living overseas, according to Diana Kerry. About 8% of those are active-duty military. Also according to Diana Kerry, American passport holders skew about 65% for Kerry, which means getting the overseas vote out could make a big difference in the election.
Get out there and tell every overseas American you know that they are eligible to vote and about these web sites. If they can't reach one of the sites, they can talk to their nearest American consulate, which will also (I believe) have ballots available.
Update: There's also the US government Federal Voting Assistance Program