‘Diva’, coming from the Latin for ‘goddess’ (see also ‘divine’), traditionally referred to a woman opera singer or, secondarily, an arrogant or temperamental woman.
Yet, over the past five years, it’s been appropriated by nearly every sector of women’s products, from shoes to dolls to feminine hygiene products. Clearly, the average marketing department thinks the average women associates positively with the idea of (if not actually being) ‘diva’.
Thinking back on it, the tipping point may have been that damn Divas Live musical special from 1998, when they were still using the term to describe singers. The more generalized usage may be older than that, but not in my (quite robust, I should think) pop culture experience.
Why is this? It’s the equivalent of men buying golf clubs by Asshole or a Jerk sedan, isn’t it? Or maybe it’s more like that cologne, Egoiste (which, I think, only appeals to a narrow slice of mankind). Or has the definition permanently changed? Have millions of marketing dollars irrevocably changed it to “strong, sexy if a little high maintenance and bitchy”?
I smell a Rebel Sell. Women are being sold on the rebellious notion of being something outside the norms of acceptability. That the ‘alternative culture’ of diva-hood is desirable and attainable. It’s apparently an effective technique.
DarrenBarefoot.com 
I’d say the popular interpretation of ‘diva’ is summed up well by this celebration of Broadway:
I always thought that Annie Lenox popularized that view with the release of Diva, but maybe that’s just me…
I’ve actually had this very thought myself.
I’m willing to accept that the term “Diva”, in the modern sense, no longer refers to difficult women (in which case would it not refer to all of them at one time or another?).
The annoying bit is more the cheapening of a term that came, in the 20th century, to mean a songstress of such talent and power that ordinary superlatives wouldn’t suffice.
Thus we hear of operatic “divas” from the 1930’s or so, pop divas in the ’80s, and now every plastic, prepackaged bimbo with an agent is referred to as a diva.
The height of irony may be the fashionable use of the word to describe country singers. Hank Williams senior is spinning violently in his grave. He’d have no truck with no highfalutin’ divas, I’ll betcha.
See, I don’t necessarily think it’s just the word diva that sells. The type of girl who buys a “Diva” branded midriff t-shirt would be equally inclined to buy one that said “Princess” or “Drama Queen” or “My Boyfriend is out of town”. It’s attitude dahling, not semantics. Also, that sort of marketing seems to skew young – does it hold as much cachet to anyone older than their early 20’s?
I’d also say the trend started in 1995 with Amy Heckerling’s excellent teen flick “Clueless”.
The word has changed. As they do. Some of it is marketing, but some of it is simply that word is short, easy to pronounce, and (at least originally) had a bit of a “bad girl” connotation, without being overtly insulting. I think the marketing push follows the trend here rather than driving it.
It has diluted the word’s superlativeness, but “awesome” hasn’t really meant “awe-inspiring” since at least the 1980s either. And we have cool new words like “karaoke” now too (plus good old ones like “skookum”). Welcome to English.