UPDATE 13th July 2016: Although Boris Johnson dropped out of the Conservative leadership campaign, which was then followed by a thinly-sourced piece saying he was still a US citizen, his appointment today as Foreign Secretary by Theresa May means that he has taken up a position considered inherently expatriating according to the published guidance of the US Department of State:
Certain policy level positions are inherently incompatible with retaining U.S. nationality. Cases of this nature generally involve heads of state or foreign ministers.
It’ll be up to State to declare whether Boris Johnson has thereby expatriated himself (and the question remains of whether he then becomes liable for the expatriation tax) but that’s certainly one way to give up your US citizenship without all the usual messy paperwork.
Boris, have you a blue passport?
https://medium.com/@holgate/boris-boris-have-you-a-passport-blue-2bec38a1e992#.ktsjpd71p
The update which is interesting: