I explained earlier this evening in a comment that Senate Democrats cannot unilaterally revoke Lieberman's committee assignments, but since then a diary calling for Reid to do just that has shot to the top of the recommend list. With that in mind, I am posting a revised and extended version of my earlier comment as a diary, so that people don't unfairly ask Harry Reid and the Senate Democratic caucus to do something they cannot actually do.
It takes a Senate resolution to strip Lieberman of his Homeland Security Committee chairmanship or any of his committee assignments. While such resolutions are generally agreed to by unanimous consent, Lieberman or a Republican would probably object to any resolution stripping him of his committee assignments. Lieberman and 49 Republicans can then either filibuster the resolution or, with Cheney casting the tiebreaking vote, defeat it. Lieberman could then switch parties and effectively throw the chairmanship of Homeland Security and majorities in three other committees to the Republicans.
For the same reason, Democrats would retain their committee chairs and majorities on the other thirteen standing committees even if Lieberman switches parties. Democrats would undoubtedly filibuster any resolution that would throw control of the committees to the Republicans. In 1953, Democrats gained a majority in the Senate mid-session, but Senator Joe McCarthy (R-WI) and all other Republican committee chairs retained their positions. McCarthy actually carried out his witch hunts during a "Democratic" Senate.
At the start the 107th Congress, with the Senate tied at 50-50, the Senate adopted S.Res 8, an organizing resolution which provided that the membership of the various Senate committees would be dissolved if either party gained a majority during that Congress. When Jim Jeffords announced he would caucus with the Democrats, the Senate had to pass another organizing resolution, which gave the Democrats a one-seat majority on each committee and gave Majority Leader Daschle and Minority Leader Lott the the authority to appoint Senators to fill the new committee seats. The US Senate in the 110th Congress adopted no such resolution. In fact, the Senate usually doesn't pass a special organizing resolution like it did in 2001. There was no special "deal" negotiated this year that the Democrats would retain de facto majority status if the Republicans gained a numerical majority mid-session; the Senate simply appointed members to committees in the usual fashion.