I have a client that wants to build a “green” home but they will not use anything that has polystyrene in it!

I am researching AAC block and poured in place cellular concrete. The poured in place makes a lot of sense because you are not hauling the AAC all over the country. Problem is I can find very little information on this subject. And even fewer people that have experience with the process.

Someone out there have first hand experience with pouring cellular concrete?

I am thinking of building forms out of MgO board, pouring and leaving the MgO as finish surface inside and out?

I couldn't find any experienced cellular concrete mixers in Pa. You may have better luck where you are building. Here are people who might be able to point you in the right direction:
http://www.cellular-concrete.com/ a leading manufacturer of the foaming agent that produces cellular concrete
Fouad Fouad chairs the American Concrete Institute's committee on cellular concrete: http://main.uab.edu/soeng/templates/inner.aspx?pid=99387 Dunno how approachable he is.
Or try the institute directly at www.concrete.org. Committee 523 oversees cellular concrete, and also AAC as a subset of cellular.
In AAC terms, you aren't too far removed from E Crete in Phoenix, or Xella USA in Mexico. My smallish AAC house fits on two flatbed semis, or ~$3k to ship the blocks from Aercon in Florida.

Do they not want to use the foam because of the manufacturing process not being green? If so, you might be able to talk them into a house out of regular block walls covered on the outside in foam, if you use recycled or factory second foam. Just stress to them that if they don't use it, it will end up in a landfill. Just my 2 cents.

If that's not it, you might even consider double stud or staggered stud walls with cellulose or fiberglass, and possibly even aerogel strips over the studs.