I believe that none other than William F. Buckley and the National Review group helped Lieberman get elected the first time; seeing him as a more conservative Democrat than the incumbant at that time.... (was it Lowell Weicker?, anyone?).
Lieberman is President Bush's best anti-impeachment card.
The only way Lieberman would switch parties is if the House tries to impeach Bush. I just don't believe Lieberman would let that happen and has let that threat be known behind closed doors.
Notice that the House dims say that impeachment is not on the table. Impeach or lose the Senate - interesting dilemma.