I have had a 23" ACD for the past 2 years, and I replaced this with a 30" ACD a couple of weeks ago (I wanted more real estate and did not want multiple monitors). I am a semi-professional photographer and I calibrate my display monthly with a colorimeter (Gretag-Macbeth i1). One of the characteristics measured by the colorimeter is luminosity. This has not changed one iota in the past 2 years. I expect the 30" to behave similarly.
Well I guess that puts the "non-LED displays lose 50% of their brightness per year" myth to sleep once and for all.