V8 isn't hardware accelerated, and probably won't ever be. That won't matter in every case, but there will always be situations in which V8 is not a valid substitute for Flash or Silverlight.
If V8 gets threading support and auto-vectorization, then if processors-graphics cards ala Intel's Larrabee take off, it may be a non-issue. Developers can just do a 3d engine in pure javascript and get roughly the same order of magnitude of performance as a GPU.