Well, the DS Lite is nothing more than a small cosmetic upgrade from the DS Phat.

The DS (regardless Lite or Phat) was released in 2004 and looking at the evolution of consoles, 3-4 years seems to be a good time to do a major overhaul.

If the rumours are indeed true, I think we will seeing more than bigger screens. It might be a totally new system from Nintendo.