Eli's solution rocks, and what I do is to disable suspicious plugins with AU Manager, then re-enable them one by one.
I had this problem because I had 2 instances of Superior Drummer 2: one kit and one latin, pus 2 EZDrummers to steal ideas for each 🙂
Even with 5G RAM this wasn't enough, and Toontrack informed me that Logic doesn't like going above 1.8G anyway.
Once I removed the EZDrummers, the song loaded fine.
peter