For those who use both GIMP and SAI like me, GIMP's layer masks are actually lots better -- in GIMP a layer mask functions like a true 8-bit alpha channel (it can be copied to and from the layer's alpha channel, actually), but SAI's layer masks are a one-bit alpha channel only: it shows through, or it doesn't. However, the SAI coders have made their layer masks much easier to use.

I was going to make a third tutorial tonight, about what happens when you use transforms in conjunction with layer masks to put an image onto a foreshortened surface, but it's gotten too late. I'll have to do that one tomorrow.