I a running version 1.4.2-2 as part of the Enthought Canopy distribution and the pixel marker (,) option doesn't appear to be working any longer. No error is returned, it just doesn't show and of the data points. Other marker options work fine.
For example:

plot(x,y,'k,')

will not show the data plots, but

plot(x,y,'k.')

works just fine. I have verified this with existing code that used to show pixel marker data points but no longer does.