there was a thread some days ago with that problem..
but it is not there anymore (at least i can not find it)...
now i had some minutes to work on it. so here my result.
i dont know if that is possible with just a shader network (i guess it is) but i wrote some mel lines that do the job..
in the scene the plane (nurbsPlane1) has an extra attribute, where you can enter the number of fields you want to have for the pixelization.. you can animate it to have funny effects.
this is a 4.0 scene.. the same scene in 4.5 does not render correctly (i already submitted the bug to alias)!!
took me some time to find that one out http://bigott.gmxhome.de/pics/reallySurprised.gif
Maya 4.0 binary example file (http://bigott.gmxhome.de/pics/expressionShaderPixelize.mb )
p.s.: just got a response from alias: it is a bug, but there is a "workaround" for it: if you change the filtering type in the file node to "mipmap" or "off" the rendering works.
p.p.s: mental ray does not like it at all.. that is logged as well.