Jonathan Tasini, a former union chief (and Hillary Clinton challenger) who runs the Labor Research association has his predictions for further union endorsements up on his blog.

He sees Edwards picking up most major unions, the AFT heading toward Clinton, the SEIU as crucial and hard to call, and this:

I don’t see Obama getting the endorsement of a single large national union—he just can’t compete with Edwards and Clinton. Short of getting the endorsements of the national unions, his goal, and Clinton’s goal, is to get large unions to stay on the sidelines in the primary and allow individual locals of the national unions to endorse on their own; in that scenario, one could imagine Obama, Clinton and other candidates (particularly sitting senators like Dodd and Biden who have long-term relationships with unions) being able to pick off locals in various states. Last point: watch where the most politically active unions with large retiree memberships end up—those retirees have a lot of time on their hands and they end up being the backbone for the phone banks, leaflet distributions and door-to-door work.