This is all true - but it is still complicated. So...in counting income you have to count stuff that is tax-free in the UK such as child benefit, income held on ISAs, currency gains on repaying Sterling mortgages, tax credits, gambling winnnings etc.
If you own even the tiniest amount of a PFIC you MUST file from 2011 (due next year) because the penalty for failure to report PFIC ownership is now $10,000 a year.
AND - don't overlook the refundable child credit and the making work pay credit. Filing might be good if you can get a refund!!