If you are a US citizen or a US permanent resident (i.e. have a green card), you must file a US income tax return regardless where you live in the world. As an overseas resident, you can exclude the first $90,000 or so of earned income and some additional if you qualify for the housing allowance exclusion. But you pay US taxes on the rest, subject to an offset of any foreign taxes paid on the same income.
Citizens, green card holders and US residents must file income tax if their income is above thresholds tied to their filing status. In 2008, a person with single filing status would've had to file if their gross income was over $8950. Gross income includes earned income as well as passive: INT, DVD, capital gains, etc. Also includes income from the US or overseas.
You will not qualify for the Foreign Earned Income Tax Exclusion automatically nor immediately. You must first pass either the Bona Fide Residence Test or the Physical Presence Test, which will entail being overseas at least 1 year among other things. You can apply the FEIE retroactively though, once you've met the tests. File form 2555 with an amended tax return once you've met the tests, but file your taxes as usual before April 15th.
If your foreign bank accounts/investments cumulatively exceed $10,000 at any point, you must also file an FBAR (Foreign Bank Account Reporting) form.
While you're here, do NOT get involved with offshore savings plans or portfolio bonds as they are taxed as Passive Foreign Investment Companies and you'll lose most of the gain.