For lighting designers who are concerned about dimmer capability with LED drivers, they should take the time to research NEMA’s latest efforts.  NEMA has developed two standards and is currently developing a third, covering different aspects of LED drivers and dimming.

Here is the scope of the SSL1 standard:

This standard provides specifications for and operating characteristics of non-integral electronic drivers
(power supplies) for LED devices, arrays, or systems intended for general lighting applications. Electronic
drivers are devices that use semiconductors to control and supply dc power for LED starting and
operation. The drivers operate from multiple supply sources of 600 V maximum at a frequency of 50 or 60
hertz.

Here is the scope of the SSL 6 standard:

This standards publication provides interface recommendations for dimming control of integrated LED
lamps intended for replacement of general service incandescent lamps operating at 120 volts, i.e., for
operation with the majority of installed incandescent dimmers. LED lamps intended for operation at 12
volts are not covered by this standard.

Here is the scope of the forthcoming SSL 7 standard:

This standards publication provides compatibility requirements when a leading edge phase cut dimmer is combined with one or more dimmable LED Light Engines (LLE’s).  An LLE, for the purposes of this document, is comprised of one or more LED modules, LED control gear (integral or remote), and a connection to the mains circuit through the ANSI/IEC base or non ANSI/IEC interface like Zhaga.

I also found this nice summary article from LEDs Magazine on the topic:

Two new solid-state lighting standards have been released by NEMA, covering LED drivers and dimming of incandescent-replacement LED lamps. The organization has also released an educational guide to energy-efficient lighting.  The US National Electrical Manufacturers Association (NEMA) has published two new solid-state lighting (SSL) standards, both directed toward designers, manufacturers and users of SSL products. They are:
NEMA SSL 1-2010: Electronic Drivers for LED Devices, Arrays, or Systems
NEMA SSL 6-2010: Solid State Lighting for Incandescent Replacement—Dimming
SSL 1 has been produced by NEMA’s Solid State Lighting section and covers electronic drivers for SSL and LED-based products. Electronic drivers use semiconductors to control and supply dc power for LED starting and operation. Topics covered include ratings, performance, and marking. SSL 1 also provides specifications guidance for electromagnetic immunity, audible noise, and efficiency calculations.
SSL 1 Working Group Leader Tom Stimac of GE Lighting Solutions said: “SSL 1 is the first in a series of NEMA SSL standards aimed at setting the foundation for quality and performance of LED systems, specifically LED drivers. LED drivers are used in every system today and the ability to verify key performance and quality aspects will be pivotal in achieving high efficiency and quality LED lighting systems.”

SSL 6 has been produced by NEMA’s Lighting Controls and Solid State Lighting sections and focuses on integrated LED lamps intended for retrofit into systems that previously used incandescent screw-base lamps. The standard addresses dimming of these products and the interaction between the dimmer (control) and the bulb (lamp), and introduces requirements to help ensure good dimming performance and prevent damage to either component.

SSL 6 Working Group Leader Robert Nachtrieb of Lutron Electronics said: “SSL 6 is the first NEMA standard to tackle head-on the importance of dimming energy-efficient LED lamps that will replace incandescent bulbs. Building on the solid industry consensus we forged for SSL 6, NEMA will continue to develop standards for other applications of LED dimming.”

Robert Hick of Leviton, member of both sections and chair of the Solid State Lighting section, said: “NEMA’s development of these standards illustrates the lighting industry’s dedication to the streamlined design and manufacture of energy-efficient SSL technology. Standardization of evolving technology is essential to ensuring harmonization across brands, exceptional performance, and customer satisfaction. With future additions to this series, NEMA will continue to identify and eliminate gaps in guidance without discouraging innovation within the industry.