The two most common speculations I've seen are that the rendering engine would be replaced by something based on Gazelle or Webkit. Personally, I think Gazelle is a much more likely possibility.
That being said, I don't think IE 8 will be teh last release of IE - I think it's more likely that it may be the last released of IE using the current codebase's rendering engine and core parsing routines. I would suspect that MS would rewrite their internals, but I would highly, highly doubt that Internet Explorer is going to go away as long as Microsoft is still writing software.
Reed Copsey
2009-03-15 20:50:00