After the urging of many Google users and privacy advocates, Peter Fleischer of Google's Global Privacy Counsel wrote in the Official Google Blog that cookies will auto-expire after 2 years, rather than in the year 2038 as was initially intended.
In the coming months, Google will start issuing our users cookies that will be set to auto-expire after 2 years, while auto-renewing the cookies of active users during this time period. In other words, users who do not return to Google will have their cookies auto-expire after 2 years. Regular Google users will have their cookies auto-renew, so that their preferences are not lost. And, as always, all users will still be able to control their cookies at any time via their browsers.
A WebmasterWorld user thinks that this is related to pending lawsuits:
Funny how lawsuits makes Goog more "user-feedback" oriented
I don't really know if I agree with that. They are the most user-feedback oriented of the big search engines from my experience.
In any event, for most people, two years is the life span of a computer, so if you think about it, unless you're using the same computer until 2038, your cookies would have no longer been in use. :)