I think the US model 60GB PS3s are the most prone to overheating, as they have both the PS2 CPU and GPU within the casing.
Later models without PS2 hardware AFAIK have cheaper Blu-Ray drives, I think those are more prone to such related failures.
I always thought it was daring for Sony to allow PS3s to fold 24/7, I would never make use of this option myself (I certainly turn off the console at night). It does however show they think/thought their hardware was relatively very resistant to overheating issues.