Although this is true, they don't gradually grow dimmer, they more or less just go all out downhill towards the end, as is usual with most LEDs (Have nothing to do with how lightbulbs work rly for the most part)
In any case, the life-expetency of TFT LCDs overall is supposedly very long. I can tell you that I have had my DS Phat for 3~4 years now (Day it came out, broke a month or so back) and I never noticed any progressive dimming in the backlights)
I've had my current DS Lite for a year and the same can be said, used every day, usually for 4~15 hours a day. They're expected to last 'millions of touches' and I've probably hit at least 1m touches already. :/