i think there's a really important difference between the very early stages of a flip-out, and when they've lost it already.
most of what i've been saying has been directed at the pre-total-flip-out stage. of course i would not try to communicate with a child who was hysterical - by that stage it is already too late for that.
whereas the conventional wisdom is to ignore, i think this is too severe, and recommend working very hard, very quickly, to get it across to them that you know why they're upset, so that frustration does not turn into blind rage etc. etc.
(and that 'i understand you feel upset because of.... - but you really must realize' formula, is really not what i had in mind. you empathize NOT by talking about their feelings (they have hardly even got the concept of 'their feelings' - that is a very grown up idea) - but by talking about the thing they're upset about. 'i'm annoyed the bike is broken too'; 'i'm upset about that as well - it would have been really nice to have a cake' etc. etc.
of course it is annoying if you're in the middle of trying to e.g. find the library, and someone starts talking to you about your feelings of frustration!! that will almost always be very annoying indeed.
more generally - i think talking to a very young child about their feelings - or their 'behaviour' is very very common, and very very unlikely to be understood.