I've often joked about building a fence around America - largely for different reasons to most (aka - keep the Americans in and away from invading the rest of the world) but have also spoken about (and used in fiction) the idea of a future American government barricading itself in to greater control its citizens.

Well, it's not exactly 'to greater control its citizens' (after all they have all the wiretapping, the 'War of Elusive but Handy Terror' and false media for that at present)  but the Senate has now approved the building of a 370 mile triple-layed 'border fence' across the Mexican border.

Senate Backs Mexico Border Fence (BBC News Online; Thursday 18th May 2006)