First: Even if Joe Lieberman decides to officially become a Republican, this doesn't give the GOP control of the Senate. This is because of the organizing resolution that was passed at the beginning of 2007, which doesn't allow a change in majority to change control of the chamber. Things were different in 2001-2002, when Jim Jeffords' decision to leave the Republican Party gave Democrats control. Why did Mitch McConnell let Harry Reid set up the rules this way, when Tom Daschle didn't let Trent Lott set up the rules like that six years ago?
(Here's the text of the 2001 organizing resolution, which allows for control to switch. By contrast, this term's organizing resolutions list off the Democratic and Republican members of each committee by name, and specify which Democrats get to chair committees. As with so many good things, I got all this in an old post from fellow guestblogger Hilzoy.)
Second: Harry Reid hasn't been forcing the Republicans to filibuster very much, even on issues where the Democratic position is popular. There was the Iraq funding filibuster last July, but that ended a lot quicker than many of us expected. Why hasn't Harry Reid been forcing the Republicans to hurt themselves by filibustering popular legislation?
So here would be a plausible explanation -- these are the terms of a deal cut by Reid and McConnell at the beginning of the session. Reid's majority isn't hostage to Lieberman, while McConnell doesn't have to filibuster popular legislation. The loser? Lieberman, who can't sell his official party allegiance to McConnell for a high price, or threaten Reid that he'll do so. Reid and McConnell also get the advantage of a Senate that isn't clogged up by filibusters and can get to the important business of letting members appropriate funds for highways and post offices.
This is obviously just speculation -- I'm not the sort of guy who's privy to much insider information. But it's the best way I can explain a lot of otherwise inexplicable things. And if this was the deal, I'm pretty happy with Reid for making it. Since I wouldn't trust McConnell to keep his end of the bargain, it's a good thing that Reid got his goods delivered in advance.