There's a discussion going on on comp.lang.postscript under the subject "Will Adobe ever complete PostScript" about translucency/alpha channels in PostScript and why Adobe hadn't put it into language level 3 and why it's difficult or not so difficult to implement.
There's certainly demand for this, but unfortunately it does seem as if any good implementation of this will require lots of memory, either to hold the entire page in at least 24 bit color in printer resolution in memory or to hold an arbitrarily large display list. Then there's the question in which color space the color blending should take place.
So far the people who maintain that it isn't difficult haven't provided any good argument AFAICT. Even if only gray-scale output is required you probably still need a full color image in memory.
I've put the first draft of an FAQ list on the web-page.
Over the last several days, I've received several interesting drawings from a Polish user of Sketch. I think I'll add some of them to the Gallery if he doesn't object. So far he's been insisting that I can do with them whatever I want...