Actually, the first $85,700 is now exempted by claiming the Foreign Earned Income Exclusion. But to claim it, you must file your 1040 each year to receive it. If the IRS sends you a letter claiming you haven't filed for a specific year, you cannot then claim the exclusion - its availability for your use disappears with their letter, and you will be liable for taxes from Dollar 1 just as if you lived within the US.
You become eligible to claim this exclusion on your earnings outside of the US either by staying outside of US territories for more than 330 days in a 365 day period or by becoming a legitimate resident of another country. There are other restrictions, such as owning a principal home in the US even though you claim to be outside the US, etc. It would be best to consult a good tax attorney who has experience dealing with ex-pats' tax issues.
This exclusion doesn't apply to investment earnings or other passive earnings like retirement benefits, just earnings from doing work outside the US.
Should you enter the US, they now have visibility of your IRS status on their computers and if the IRS is looking for you hard enough, the Immigration officer at the port of entry can retain your passport until their questions are answered to their satisfaction.
It may be best to fess up and clear it up. With penalties and interest piling up, it might hurt, but trust me it will be even more painful if you choose to do nothing and let the clock keep running on your mess. Who knows? The US gov't need $$ more than ever right now, and they might settle for a fraction of what they may be claiming is due now, or agree to a payment plan.
Your cheapest option right now might be to invest in an hour of a good tax attorney's time.