Orr... since they're like $5, instead of paying someone to stand over an open flame and torch the screen, then scrub and scour it and end up with $10-$15 in labor, you could just buy new ones ;)
A neat trick taught to me by one of the heroes at the local espresso maintenance and repair shop:
Really nasty dispersion screens can be cleaned with a gas stove or a propane torch. Hold the dispersion screen in the almost invisible part of the gas flame until it glows red and stops smoking.* After it cools, simply brush off the ash, rinse, and re-install. It'll change the finish colour of the screen, but it works really well.
*All the caveats about open flame and red hot metal apply, if you can't figure out that you're supposed to be using tongs or pliers or some such and be really careful, maybe you should be buyihng new...
yikes! its really important to clean the portafilters and the machine daily. you will notice a big difference in taste so be sure to sample the espresso afterward...you are in for a tasty treat!