See thread
here (scroll down about halfway until you see my pretty pictures).
One updated note, is that if you look at your .ase file in a text editor, you'll see three material references for each material:
MATERIAL_NAME
MAP_NAME
BITMAP
MAP_NAME is actually unused, so it doesn't matter what value it is. MATERIAL_NAME is only parsed by GtkRadiant 1.4.0 as the "editor" image... it only points to the location of the texture/shader for the editor only. BITMAP is used by GtkRadiant 1.5.0 and Q3Map2 to point to the actual location of the texture/shader. So there is a bit of inconsistency as far as how everything works.
For most models, it should be simple enough to make the shader = texture location.
So if your texture is located at "models/spd/mymodel/texture.tga", your MATERIAL_NAME and BITMAP values should be, "models/spd/mymodel/texture".