Here's a (stupid) thought: what if the whole NARAL endorsing Chafee move is an attempt to motivate the state's (small) conservative base to fund a challenger? For Chafee to get NARAL's first 2006 endorsement and speak at their convention might be a bit much for Rhode Island's hardcore Republicans to bear. The Club for Growth, now headed by Specter's primary challenger Pat Toomey, is publicly considering funding someone, and this would be the perfect outrage to rationalize their entrance. And weakening Chafee in the primary would soften him up for the general, an especially important task considering that Langevin and Kennedy both begged out and their Democratic replacements are weaker.
Anyway, probably not true, but if their really was a genius this devious within NARAL's walls, I'd be quite impressed.
Update: First, more serious post on the subject here. Shorter me: NARAL's being stupid. By the way, Matt Singer thinks that judgment is wrong, wrong, wrong. Thought you should know.