s > sh in AmE

s > sh in the context of st, str (permitted clustered)
student > shtudent
straight > shtraight

The cluster "sr" is not permitted. However, it occurs in certain contexts

Sri Lanka > shri lanka
That's right > tha'sh right (in churhes)

grocery with the  variant sh
luxury with the variant sh
anniversary with sh
http://www.youtube.com/watch?v=WgKmpQGclmI

http://literalminded.wordpress.com/2011/09/06/shtraight-talk/

http://literalminded.wordpress.com/2010/03/17/affricate-identity-crisis/