Actually I created my terrains exactly that way. When I tried DDS textures, I was creating the mipmap chain with the Ati Compressonator, I put the textures in the level folder and in MED I was setting them up as external textures.
But like with (internal) .tga textures only the first texture is rendered with mipmaps. In my terrains the first texture is usuall a gras texture.
I was making some screenshots.

This one shows the terrain with .tga textures (mipmaps created in MED):



This here shows another terrain with DDS texture. While the gras texture is rendered with mips, the roadtexture in the background doesn´t show the mips.



For comparison here are two other screenshots with the modified template code. The road and the rocks are blurred in the distance.





But anyway, if noone else has this problem it is maybe my videocard or something else in my configuration. But the strange thing is, that normally mipmaps do show up. It´s not an urgent issue for me (I´m just testing and developping my skills at the moment), but I wanted to inform you about this and if there´s an opportunity, maybe someone could have a look at this on a different system.