Migrating to Cell would be a bit of a hassle. Not an x86-type hassle, because it wouldn't break support for current software (I think...), but it would force developers to learn how to optimize software all over again before any performance gain could be achieved. Remember when the G4 first came out, and it was pretty much exactly the same as the G3, because no apps were optimized for AltiVec? It'd be pretty much the same deal all over again. And AltiVec is hard enough to program for as it is; Cell is like AltiVec^2 as far as difficulty goes.
The reason the Cell is so powerful is because it uses a bunch of separate mini-processors, called SPEs (synergistic processing elements), controlled by one full-blown PPC. You can think of each of these SPEs like a standalone AltiVec unit, except and this is important for any Cell-based Mac theories they are not based on AltiVec. They're not as flexible, and they're not as powerful. They do what they need to do in the PS3, and they do it damned well, but that doesn't mean they'd make good desktop workhorses.
I wouldn't be really surprised to see some Cell-type architecture show up in Macs eventually, but I think it's a bit early, and I would expect to see a much more desktop-centric variant of the design that anything we've seen so far. The interesting thing about the Cell is that the design itself is very flexible you could plug in as many SPEs as you want, and you could, in theory, use a completely different kind of SPE that anything that's currently been discussed. That's why I keep referring to it as an "architecture" rather than a "chip". The Cell as we (barely) know it will never be used in Macs; that much I feel confident in saying. But we might see a Mac-worthy processor that follows pretty much the same formula at some point. Although, whatever the specifics of the design, I think it would still require major re-optimizing of software.
Also, the Cell isn't really ready to go yet. By the time it is, I expect to see the current G5 advance far enough to make a move rather silly. By that time, we should have 2-core (or maybe even 3- or 4-core) G5s (and even G4s), which would present a big leap in power and a virtually seemless boost in performance (most MP-aware apps should be MC-aware right out of the box, so programmers won't need to re-optimize everything, IINM).
It's anybody's guess as to when we'll see multi-core G5s, though, really. From what I was hearing about last year, I honestly expected them by now. I still expect them by the end of the year my feeling is that the whole 2.7GHz G5 was purely an interim upgrade to tide us over until the real stuff is ready. I could be entirely wrong, of course. But the PS3 isn't coming out until next year, and there's no real reason to think the Cell will be finalized very long before then.