Okeedoh, "whistle register" is a term for the extreme upper range past high falsetto, which is completely, more conventional terminology, a 'head tone.'
Falsetto is not rare, the head tone, rarer, but not extremely rare. The higher up the register in the head tone, there is an acoustic parallel to how a whistle works -- i.e. the vocal chords are very little involved, and a kind of overblowing, like a flute or whistle, with diligent control of the air column, is producing what is actually a vocal overtone, the mouth and head as resonator, the chest also not in the tone production picture -- but I've never heard it called nothing but
falsetto or ultra high pitched
head tone.
http://en.wikipedia.org/wiki/Whistle_register
Everyone has a falsetto, it is basically a 'head tone,' and the timbre change is less noticeable in women's voices. Too, classical training would have a singer transition seamlessly from the combo of chest / head to head tone without a dramatic shift in color -- the classical technical 'secret' to a smooth sound over a range of three or more octaves.
Males tend to lose their falsetto because they simply stop using it, often not long after their voice changes, and too, it is lost due to social notions of what 'manly' is.
(David Daniels was an undergraduate where I attended school, and I recall his teacher speaking about him with another faculty member, rolling her eyebrows and saying, "He has this falsetto he wants to use." LOL. She must have capitulated, and Daniels later got great training and is now a world-renowned much in demand counter-tenor.)
Handel ~ Rinaldo; Cara sposa, amante cara; David Daniels
Training in classical singing usually does not at all focus on developing the falsetto in males, where the females, mainly the high-register coloratura sopranos, do routinely learn to use this extreme high head tone.
For one reason or another, the call for falsetto male singers died a quick death after the Castrati countertenors were replaced by women in the opera, and since then, a bit of it here and there in a male role, usually for dramatic or eerie effect, is called for and written. That flagolet / overtone / whistle effect is very rarely called for, I would wager not until the 20th century, and if so, specifically written for a singer with such a capacity.
Coloratura Sopranos use it quite routinely, the music written for them planning upon that capacity, again usually without such an abrupt change in tone color:
Falsetto and that 'whistle register'
could be readily learned and achieved by many a competent singer, but that sound, so far, is not called for with any frequency from classical composers. I think you'll find those eccentric uses more in pop music, the singer either jamming or writing their own music to show of, and use, that part of their range.
Pop song writer or classical composer, neither want to write something to be possibly performed by any number of performers, and throw in a trick, or set it in a range, where only a handful of performers could pull it off... that severely limits the possibility of both performances and sales of the sheet music.
Coloratura Sopranos, "The German nightingale" Erna Sack, and the French "stratospheric colaratura" Mado Robin.