How to make clouds?

I have seen pictures of some very realistic clouds made using Blender 2.5. I’d love to have a go. Does anyone know of a tutorial for this? Or even some hints?

Don’t know about 2.5, but there are some cloud generator scripts out there:

You can also google “Blender cloud generator”

hope that helps

probably not what you are looking for but the example renders are quite good

Thanks, both. I shall scrutinise these to see if they give me any hints for doing it with the new volumetric materials.