Reference for von Neumann algebras coming from a group algebra twisted by a 2-cocycle?

You can consult the article

Bédos, Erik
On Følner nets, Szegő's theorem and other eigenvalue distribution theorems.
Exposition. Math. 15 (1997), no. 3, 193--228.

and this recent article:

http://arxiv.org/PS_cache/math/pdf/0605/0605145v2.pdf

Finally, I believe that the construction is first appeared in

Zeller-Meier, G.
Produits croisés d’une C*-algèbre par un groupe d’automorphismes. (French)
J. Math. Pures Appl. (9) 47 1968 101–239