With the withdrawal of Jan Ullrich, the 2006 Tour de France is the first not to include any previous winners at the start since 1999, the year of Lance Armstrong's first victory. Of course, since Armstrong won for the past seven years, it isn't surprising that there is a dearth of former winners still competing in the sport (1996 winner Bjarne Riis is the boss of Team CSC, but he'll be driving a team car, not riding). The only former runner-up in the field is T-Mobile's Andreas Klöden, with CSC's Bobby Julich the only third-place finisher. Because Klöden hasn't impressed since and Julich is beyond his prime, the odds are excellent that we will see an all-new podium this year.
But perhaps more historically, has there ever been a Tour de France wherein none of the previous year's top five took part? I don't have the data to do the research easily, but I suspect that 2006 may be the first. At the very least, we probably would have to go back several decades to find a similar instance -- maybe the 1947 Tour, which was the first since 1939 thanks to World War II.
This could be a watershed moment, a complete changing of the guard. And in the face of the Operación Puerto doping scandal, that is just what pro cycling needs right now.
07/16/2006 - UPDATE - I followed up on my hunch about the 1947 Tour. It turns out that Rene Vietto, 2nd place in 1939, not only raced in 1947 but finished 5th. That poor guy might have won a Tour or two if not for WWII interrupting his career.