Sounds like brightness and/or contrast settings may have been reset to native in the upgrade. What happens when you build a new custom profile with your former parameters?
Well, it can be, but I think is a Mac OS issue. I´m using a 30" Samsung SyncMaster 305T (it´s not a wide gamut monitor and it comes factory pre-calibrated). I really like the monitor, specially the anti glare screen (much better than the Dell 30"). The difference I see in brightnees and contrast switching between Lion and Leopard happens using the same original icc profile (name and creation date). So, I don´t know where is the problem. I´m keeping Leopard because of Rosseta based app.