Clement Mallard has made an amazing real time shader for interactive texturing. Check out the video for more details:

From the vimeo page:
Realtime Mix8colors for Softimage with map lookup.
- Blend textures together
- Paint the influence of textures with weightmaps or vertexcolors
- View several weightmaps at the same time
- Blend textures with a weightmap
- Use vertexcolors as clusters