screenpos = (mul(matModelWorldProj, pos).xy*0.5+0.5);

or what should also work is VPOS: http://msdn.microsoft.com/en-us/library/windows/desktop/bb509647(v=vs.85).aspx#VPOS

Lite-C Tut. 1 Lite-C Tut. 2 Lite-C Tut. 3 (en)
Shader Tutorial (ger)

http://www.slindev.com