Politicians who decide against running for office because they know they can't win almost never admit it. Unfortunately, they have to provide some reason, and that reason is often vague. Here's Joe Lieberman's today:
The reason I have decided not to run for re-election in 2012 is best expressed in the wise words from Ecclesiastes: “To everything there is a season, and a time to every purpose under Heaven.” At the end of this term, I will have served 24 years in the U.S. Senate and 40 years in elective office. By my count, I have run at least 15 full-fledged political campaigns in Connecticut.
I believe this is what philosophers call "begging the question."
As to his future plans, Lieberman is vague:
I do not intend today to be the end of my career in public service. Having made this decision not to run enables me to spend the next two years in the Senate devoting the full measure of my energy and attention to getting things done for Connecticut and for our country. I will keep doing everything in my power to build strong bridges across party lines -- to keep our country safe, to win the wars we are in, and to make sure America’s leadership on the world stage is principled and strong. I will keep doing everything I can to keep our economy growing and get our national debt under control, to combat climate change, to end our dependency on foreign oil, and to reform our immigration laws. And when my Senate chapter draws to a close in 2013, I look forward to new opportunities that will allow me to continue to serve our country—and to stay engaged and involved in the causes that I have spent my career working on, and that I care so much about.
I'm guessing he has a sinecure at a foundation or think-tank dedicated to promoting hawkish foreign policy or centrism. The right-wing version of this career plan would be an AEI fellowship where he will produce a book and a series of op-eds on the theme I Did Not Leave The Democratic Party, The Democratic Party Left Me. The left-wing version is a Brookings fellowship consisting largely of providing quotes to the mainstream media bemoaning the decline of bipartisanship, punctuated by service on a large number of blue ribbon panels. Or, again, possibly some kind of foreign policy-centered think-tank.