The new auto-upgrade model Mozilla has adopted for FF 4.0 (and that Chrome has had for ages) makes upgrade cycles, for all practical purposes, negligible.
Chrome 6.0 was released on September 2. Since then, Chrome 5.0's market share has dropped to nearly 0%, with Chrome 6.0 picking up all of 5.0's share while also absorbing a good amount of IE's share.
If Microsoft instituted an auto-upgrade path for IE and released updates with more regularity, this would slowly become a thing of the past.