I’ve got the ARB multitexture working but I want to fade, for example, a grass texture into a dirt texture between vertices.
Is that not a job for multitexturing? Do I have to make two passes and render the landscape twice using single textures and specifying alpha with glColor4f?
Yes you can do this with multitexture I think you need to use the env_combine extension so you can take vertex attribute directly into the second texture unit to apply the INTERPOLATE_ARB between the previous unit and the current texture: