I thought there were tax treaties, basically you don't pay taxes on the first $100k if you paid into the national tax system of the country you are in. I can imagine FICA taxes though might be difficult. Maybe US social security would deny benefits if you claimed them without any contributions and residence abroad.
So, if you happen to be an expat in a higher tax country (which is certainly Europe), then you won't owe taxes in the US. You still have to file, which can cost $500 for an accountant to make sure everything is correct, but owing taxes isn't really an issue.