Official release of SketchUp 5 is due in mid-summer 2005 -- end of July? You get a free upgrade by buying (or upgrading to) SketchUp 4.0 now. If you did so prior to 1 May, you're outa luck and gotta pay US$95 for the new release.
@Last Software is showing SketchUp 5 at the AIA National Converntion and Design Exposition (booth # 4427) May 20-21. A beta is expected in 2-3 weeks for Macintosh and Windows.