Regarding John's question, is "digital" amplification the way forward, the answer, imo, is clearly yes.
Why did I put digital in inverted commas? Because it's a bit of a misnomer. Class D is so called no because D stands for digital, but because it came after class C, which came after B, etc. Class AB that most of us know is a cross between class A and class B.
The other reason it gets called digital is because the output switches between high and low (it has two possible states, like a digital system). The distinction is subtle, but class D (and T) are "switching" or "PWM" (pulse width modulation) amplifiers.
Why are they the future? Because the future of hifi in the mass market is more living-room friendly, and more channels. With LCDs and plasmas, the TV no longer takes up half of a british living room, so the hifi isn't allowed to either :SWMBO: The very high efficiency (~95%) of PWM amplification makes them good for this, because power supplies can be smaller, little to no heatsinking is required (unlike my Class AB amp with several square feet of heatsink per 100W channel) so the component can be made very small. In addition, cheaper, smaller switchmode power supplies can be used because if the psu switching frequency is tied into the amplifier's PWM frequency then the output filters of the amplifier will remove both. Thus a manufacturer can make a tiny little box (by class AB standards) that will give you 5x100W for real.
Many companies are going that way. Bel Canto in the high end famously gave up their life-long champion Single Ended Triode amplifiers in favour of the Tripath Class T (a variation on class D, mainly a marketing thing), and Sony have released some high-end S-Master class D amplification. Looking more widely, Harman Kardon are using the D2Audio PWM module, and there are various others on the market (sharp and yamaha have their own, Michaelab is working on his LCaudio based amp, etc).
The other side of it is that for the costs involved, class D and T can (but do not necessarily DO) sound extremely good. Better or worse than "traditional" amplification (worth noting that Class D came about in the 50s) is down to taste of course (much like solid state vs valves).
For the mainstream market (mini/micro systems, hometheatre, etc) within the next 3 years class D amplification will be the majority of products sold. In hifi as we know it, of course there will be manufacturers who will stick by class A, AB and so on, just as there are manufacturers who stick to valves. However, I think we will see more and more class D amplifiers here as well.
The other side of "Digital" with regard to class D amplification is that they can be and often are microprocessor controlled with DSP (D2Audio, Sony S-master and Tripath Class T all are). This means that any analogue input is converted to digital, processed, output to the output drivers, where the "conversion" to analogue is essentially a by-product of the amplification process. Of course, this means that many of these amplifiers can take a digital input. Sony, unsurprisingly, have tied their S-master amps to the bit rate of SACD (2.8 something MHz).
The comments previously about simplicity being a good thing for sound... well, it doesn't get much more simple than that. The bits on the disc are essentially driving the output drivers of the amp. All you need is an SACD pickup, feed into the output drivers, filter the HF off and there's your sound.
Of course, the worry must be that several manufacturers will use the same modules (eg D2, Tripath) and they will sound the same. There is some tweaking that can be done (eg the Bel Canto implementation of the Tripath was rather different to the Veritas), but that varies with chipset (the D2 is almost untweakable in hardware terms. Can do lots with software on it though). Of course, power supply design and quality will remain a significant issue. What will probably result is that different amplifiers using the same chipset will have a similar sound, much as two push-pull valve amps using the same valves will also have a similar sound. The other details will still have an effect.
