As long as the glass tube remains intact, no mercury is released from a CFL when it reaches its end of life. In some rare cases, when a fluorescent bulb reaches it end of useful life, the arc contained in the tube may elevate the temperature of the housing plastic near one end of the tube. This elevated temperature, although it is short lived, may produce some limited smoke and odor from the plastic housing or coatings. Although the odor may have a bad smell it is not harmful - it's just the burnt plastic. The smoke does not contain any mercury as long as the glass tube remains intact.
ENERGY STAR qualified CFLs are designed to meet UL standards, which require the materials to be self-extinguishing, and it is the nature of fire retardant materials to exhibit some deformation or discoloration when acting in protective mode. Read more about what to do if your CFL smokes.
Read our FAQ on how to dispose of CFLs.