Further to the foregoing: I am reminded of an off-Internet Listserv discussion among members of the tax-crime defence Bar and the tax-compliance (call them the "OVDP-promoting" Bar) on how Big Data -- and TECS, the Treasury Enforcement Communications System, that records airline reservations and cross-border movements
http://www.irs.gov/irm/part5/irm_05-001-018r-cont01.html -- can or might coordinate information about dual nationals and lead to writs
ne exeat republica http://www.irs.gov/irm/part5/irm_05-021-003.html (lots more on this stuff with an Internet search; Forbes Magazine has covered (and indeed instigated) much of the Taxation to Avoid Expatriation and the enforcement story. LinkedIn groups are one site where this stuff is discussed publicly although many of its members can be said to be touting for business by carrying IRS scare stories.
As IRS lawyers have told me (in relation to cases that shock the conscience like those where taxes are more than 100% of income -- typically the old dot-com stock options cases and the Carter-years no Foreign Earned Income Exclusion cases) the IRS isn't looking for such noncompliant people and deals with them "when they are brought to the IRS's attention". As of course they must.
The story as of 2003 (quoted in the Britishexpats forum in 2004) is here, but the law has changed somewhat:
http://bit.ly/1cl3eJ3 (the thread concerns diplomatic immunity)
Flyertalk has been discussing "emergency travel by a US citizen" with a foreign passport:
http://bit.ly/1FK14fx Note, however, that the "gate controllers" are the carriers -- air, rail, bus -- who will not board anyone they know or have reason to think is an Amcit without proper documentation.
That said, there are millions of persons around the world with US ancestry, even a US parent, who do not themselves have a claim to US nationality. Or for whom a successful claim would require resolution of doubtful facts: parental residence, place of birth, biological parentage (DNA), and so on. Decisions of the State Department and UCIS can be appealed on the facts and the law, and precedent decisions can be found online:
http://www.uscis.gov/laws/precedent-decisionsSome jobsworth
https://en.wikipedia.org/wiki/Jobsworth could perhaps cause trouble. But there's no promotion for him or her in doing that.