Probably when BHO speaks the Queen's English with a plum in his mouth!
I find it unbelievable that after USA have made so many decisions specifically to "forget" their British heritage they continue to cling to such relics as feet, pounds, horsepower . . . .
Having grown up with imperial then having to go through the "migration" stage & finally enjoy the ease of the metric system for several decades now, I just can't understand why a nation that still claims to be the most powerful on the planet doesn't have the political will to make it happen.
Just for good measure (& make you smile) I have to have a dig at the Poms. Because they didn't mandate a "metric only" system (as Australia did) they're still burdened by a dual-use system although it would seem EU membership is reducing that. (If a Brit can provide more recent data I'd like to hear about it)
When are the rest of you folks going to get with the program and switch to Imperial??<sigh>