The disappearance of the traditional amplifier

As Chad mentions it is cheaper - it is easier to build a higher performance amp with limited bandwidth powering one driver than the same quality performance with very large bandwidth driving several drivers as well as a lossy passive crossover. Usually when something is both cheaper and better in performance it tends to win out in the market place. I suspect active speakers will win out eventually if not for performance but more likely because of lower cost for similar performance.

These powered monitors got tubes in 'em? ;^}

What goes on in the pro marketplace does not necessarily translate to the consumer marketplace. I think that powered speakers have a chance in the consumer marketplace for the reasons Shadorne mentions and also for the fact that they are more neat and tidy for a home decor--no big boxes cluttering up the living room. B&O has been quite successful with this approach, for example, in the less-than-high end, and Meridian and ADC have had some excellent designs using this approach that I think most of us would consider high end. I doubt, though, that those of us in the lunatic fringe (and I include myself in that group) of perfectionist audio will completely embrace the technology, as it limits our choices (no tubes, as Rockadanny gently points out) and ability to "upgrade", and the perception (right or wrong, and Meridian's and ADC's powered speakers make a strong case that perhaps the perception is either wrong or overrated) that constraining an amplifier to such a small, vibration-prone space will be deleterious to the ultimate sound from the speaker.

Rcprince brings up the typical objections to powered speakers. However, when carefully examined, the advantages of active speakers with active crossovers will far out weight the drawbacks. And, what is too frequently overlooked is that not all active speakers are powered speakers with built-in amps. The active speaker with outboard amps provides both the superior performance and the choice of amps, cables, and the option of experimenting all you want.

vibration-prone space will be deleterious to the ultimate sound from the speaker.

It certainly would if you used tubes. In that case you can still go active but they would need to be 'outboard' amps dedicated to each driver (this would still make the design active and is actually how active speakers started)