It is on their bigger samplers, but only in limited amounts (mostly the morphing filters, but the other filters are smaller versions of them), but I think the Audity 2000 has them in full amounts as well as the Morpheus.
What is it? Other filters are two-dimensional: filter cutoff and resonance. Emu's z-plane filters add other dimensions to this, so that the filter basically is a cube, ie it can move in three dimensions (Z=depth, hence the name). This means you can have filters that move a sound from a lowpass rumble to some kind of highpass or bandpass filter, with any way inbetween. The problem is that this is really hard to control to get musical results, and in the Morpheus you get a couple of hundred filter paths that you can move along with LFOs, controllers etc. It doesn't sound like anything else really, and people with Morpheuses seems really happy with them. It depends on what you want to do with it. A sampler is much more versatile, and to a limited extent you can do the same thing as the Morpheus with the EOS samplers, but also a lot more. But if all you want is airy pads then a Morpheus is better (or a Wavestation, or a MW XT, or maybe even some kind of rompler).
Emu did have a long interview with the people behind the z-plane technology on their old site, but I don't know if it's still there. They can probably get if for you if you ask them nicely.