According to www.motortrend.com or oct. issue p.28. BMW sources indicated that they are considering using a V-10 version of the BMW.Williams F1 V-10 in the next M5 (2006) because its gonna be a major horsepower war between BMW & Merc. Merc announced that their next E-Class AMG will be running a supercharged V-8 5.5L and BMW don't want to lose. Also sources indicated that the V-10 will be fed through a F1 six-speed paddle-shift. As for the rumor of a V-10 M6, BMW said NO, since styling of the 6 series is completed and it can't accomodate the V-10.
I hope all of this is true, because if there's a major horsepower war coming, I am getting my check book out and wait.