Color Separations are NOT handled by the artist, they are done here at the print shop. The artists usually CREATE their art in color LAYERS (and LEAVE them in layers), which really helps me when I COLOR SEPARATE the design -which is an entirely different process than creating art (Don't do this at home, kids).
is100, the design you called out above is on a baby blue shirt. There is usually not an opacity issue when printing on lighter shirts (baby blue, white, creme, silver). However, your design is on a very dark garment. This would require a base(usually white) printed down first to give the lighter colors some 'pop'.
tjschaeffer, yeah, that design was pushing the limits to the detail we can hold. Again, it was on a light garment and we can push the envelope a little more there. The real problem is when the garment is dark, and a CHOKED base is required. If an element in your design is 2 pixels wide and we have to choke it back 2 pixels, guess what happens?