Response to this
A most interesting post, and one that certainly points out the fallacy of 'high culture' and 'pop culture'.

As someone whose primary artistic involvement (and certainly field of knowledge) is in music, and specifically singing, that's the angle I'm going to reply to this from.

Divisions into low, high, intermediate are just as artificial as dividing the human voice into soprano, bass, tenor, sort-of-alto-but-with-bits-of-mezzo.

See posts passim on my experiences of such pigeonholing. I asserted a while back (though not, if memory serves, on this blog) that the natural median of a population's vocal range (covering each gender) is roughly bell-curve shaped (fairly obviously, males and females have their own bell curves, roughly an octave apart). This is by definition one-dimensional, and there will be variations in the scale of the deviations from such a point (read: some people have bigger ranges than others). The general point, however, is that most choirs divide each gender into two, and a bell-curve implies that is extremely inefficient - there'll be plenty of singers falling between two stools and not quite suited to either, and I'm one of them.

Even within a given voice type - or fach, as I believe is the term in classical singing - there are more than a few variations. So many, in fact, that there are sub-sections of fachs (sometimes sub-sections of these, too), best conceptualised to this audience as sub-directories within a paper or electronic filing system: C:\Soprano\Lyric\Coloratura anyone? (And that'd be a perfectly valid description, and I know several people who fit into it.) Even within such seemingly vanishingly small pigeonholes there seems to be variation, but perhaps not surprisingly there's also uncertainty over which hole a singer should be placed into.

So when it comes to writing for others, I don't bother. Take NI, whom I spoke to last night. I know that she can comfortably sing Ab3-A5, and from last night's conversation I also know she's happy to belt (in the musical theatre sense) as far as A4. Some would take this information along with a few other elements (particularly timbre), put NI in a pigeonhole, and in effect that information is lost, for NI will be placed in a pigeonhole with fellow singers possessing voices subtly but substantially different from her own. (Many won't have her musical theatre background, for a start.) Furthermore, if trained with an emphasis on classical performance, as NI is, there won't be an incentive to grow outside the demands on this pigeonhole. That is the problem I feared, and as a composer herself NI is likely to appreciate this. Instead, my approach is right, I can write melodies up to A5 and down to Ab3, and in the lower of these two octaves I have a choice of two distinct timbres. That's it. No loss of information.

From my perspective, the fundamental division seems to be, if they have a great deal of vibrato, they're a classical singer, or at least will be seen and treated as one while performing popular music; the reverse applies. To which I reply only with the early music singer Dame Emma Kirkby, whose performance at the university concert hall is the one cultural event I really wish I'd been to last year but didn't. (Swap that with the Medics' Revue...)

Musical theatre rather blurs a lot of these lines anyway. Leaving aside the question 'where does musical theatre fit in this continuum of culture?', there's a separate but ultimately not-unrelated quirk regarding musical theatre singing itself. Invariably, a given character will have various demands and some - but not all - of these will cover areas classical training eskews, the aforementioned belting being the primary example (and one that in itself makes some classical teachers wary of musical theatre, as it's a threat to their perceptions of what is 'healthy' singing, and is dangerous done badly - I shall have to speak to NI more about this). And yet, in spite of this, there's enough that is more in common with classical singing than anything else. (The role of G(a)linda is a particularly good example, and to a lesser extent Elphaba too.)

As has been pointed out before in comment boxes, there is practically a divide in pieces for female voices, with most of the soprano repertoire being classical works (and a corresponding shortage of classical works for lower female voices, unless one counts those written for boys?). That, in turn, probably explains the existence of the crossover market; it has effectively created a place for voices that do not fit into the pop mould, which seems to be similar to the musical theatre mould (leaning towards tenor and alto voices) but rather more strongly so, to have a pop career.

The Three Tenors arguably worked at bringing high culture to the masses. (They were certainly the most successful part of Italia 90...) The myriad crossover singers (and it is interesting that the original post mentions specifically Katherine Jenkins, a mere mezzo-soprano, the only female NotSoprano I can name with a crossover career, and frequently quoted in the press as though she were a soprano) are doing nothing of the sort. They're doing faux-operatic covers of pop songs for an audience probably closely matched to the Hell's readership.

In short, vocal music alone is so immensely pluralistic in its nature that a high culture/pop culture continuum is largely useless. However, by the mere assertion that it exists, it becomes perhaps closer to existing, especially when so many of the attempts to reconcile the two are such abject failures.