According to a SpeedTV.com article, 2006 will mark Michelin's last year of participation in F1, leaving only Bridgestone to supply tires to every team. The FIA had been pressing for a single tire manufacturer rule, although most of their plans pinned the change on 2008. Michelin cites philosophical differences as the reason for their departure, although that sounds like a polite way of telling the FIA to pound sand. The FIA and Michelin never came to a satisfactory resolution following the 2005 USGP, in which the 14 Michelin-shod cars (out of a field of 20) boycotted the race due to safety concerns. That incident put a permanent scar on the reputations of both Michelin and F1, and everyone still blames everyone else for failing to resolve the situation gently.
Michelin tires were on the winning car in 18 of the 19 races in 2005, although this past season was something of an enigma. Mid-race tire changes were prohibited except in extreme circumstances, and Michelin clearly adapted to this more effectively than Bridgestone. New rules for 2006 mark the return of mid-race tire changes and while that decision would seem to favor Bridgestone, the loss of Michelin is a significant one. Without any competition, one wonders how much effort Bridgestone will put into mid-season tire improvements.