It's the quality of the unit.
Some LEDs can lose a decent amount of light in the first HOUR of use.

The life span which Cree (for example) claims basically means that at that point, their fixture will be emitting at least 70% of the amount of its original brightness.
So yes, they will dim- in varying amounts.

The nice thing is though, is that after the 50k hour time frame, it's not going to burn out. So if you happen to not need full brightness, you can keep using the bulb for years more.

But it's a good question. How can we really test the 50k hour claim? The way I figure, I go with the trusted companies- Cree, Nichia, for example.
Even then though, someone can be using Cree's LED, but if they build a crappy fixture around it, all of Cree's quality will be practically meaningless.

Actually the degradation is much faster than the CLAIMED 50K hours.
Check for Nick Hill's research on this topic.

Many or most of the white LEDs use a phosphor. It is the phosphor that degrades with time and use.

But all lamps do this to some degree. The phosphors in fluorescent lamps also degrade and their light declines over time. Incandescents' envelopes darken as the tunsten boils off the filament and deposits on the cooler envelope, and this blocks some of the light output. (This effect is reduced but not eliminated in halogen lamps).

Falling light output is inevitable with all lamps, to one degree or another. Perhaps it is more significant in LEDs because of their very long theoretical life.

However, note that the 40k or 50k hour LED life rating is always an average. LED lamps may fail prematurely, just as compact fluorescents do. Other than the outright cr@p that you buy on Ebay and at fly by night online retailers, we are probably not seeing too much of this early failure yet because until recently LEDs have been expensive, limited production items.

As LED production is increased and transferred to China and other low-wage nations, and as price competition heats up, cost cutting will become more common and quality will decline. Then we are likely to see more premature LED failures. Consumers will complain about 50k hour rated LEDs that last only a few thousand hours, even though some of their other LEDs last 100k hours or more. That is the way people are. :)

They will dim over time, but lets keep the time frame in perspective. Good old inefficient incandescants have the shortest life span measured in months and they deteriorate over time. Compare that to LED's which will last years or decades depending on use. And you have the added benefit of LED's using relatively little electricity for an equivalent amount of light.

There are several factors that can reduce the life of an LED light over time. As with any type of electronics or circuitry, the life of the total package is determined by the life of the weakest part of the package. If the LED driver, capacitors etc. in a LED luminaire are cheap then they will be the first component to fail, even though the LED itself would still light up. Over driving the LED with high current (that's a big one as it leads to decreased life and shifts in color temperature) or testing at lower than normal operating temperatures will also lead to exaggerated claims in lifetime. That's why buying from brands and companies who are reputable is important, especially considering the major investment that LED bulbs are. I wouldn't be surprised to see a lot of the early no-name bulbs fail way before claimed lifetime due to the fact that they were being over driven.

polar sean is 100 percent correct.