No one's really sure. Up until a couple weeks ago, the prevailing theory was that BMW will introduce the new model ('E60') at the Frankfurt Auto Show next September (2003), with arrival at US dealers in early 2004. But some recent spy shots--if accurate (and they may well be bogus)--have suggested the platform is further along than thought before and that BMW may be rushing it out the door to compete with Mercedes' new E-class (due very soon). These "new" rumors now say BMW may feature the car at the Geneva show in March of next year, with arrival in Europe shortly thereafter and in the US by late summer/early fall. Some early pictures (heavily computer-aided) can be found at: http://www.bmw2002.co.uk/news.htm , but no specs have been leaked yet (aside from the presumption of the valvetronic V8 from the 745 and likely 6-cylinder versions as well).
We'll see, but personally I'd stay away from the first model year to assure they've ironed out the bugs (I've already read more than a couple nightmare stories about the new 745's electronics going berzerk).