This is definitely possible now. It's just a matter of using the right density shader. Some decent results have been achieved using just a single default density shader and adjusting the warping settings in it. You could create a nice, complex network of shaders above that to get better results.
I think creating a good, realistic-looking global cloud model would take some time and involve multiple cloud layers, which you'd have to blend smoothly and realistically together, but I do think it's possible currently. It would just take a lot of work. If you don't need to see your clouds from space *and* from the ground, then focus on the large-scale stuff. Usually only 1 or 2 layers would be necessary for that.
- Oshyan