Are any special certs or licenses needed for Snow-ski equipped planes?
From the FAA manual:


  Title 14 of the Code of Federal Regulations (14 CFR)
  part 61 does not require specific pilot training and
  authorization to operate skiplanes; however, it is
  important to train with a qualified skiplane flight
  instructor.
  
  Since most skiplanes operate in a wide variety of conditions,
  such as landing on frozen or snow-covered
  lakes and sloping glaciers, with varying qualities of
  snow, it is important to know how performance is
  affected. Use the performance data provided by the
  manufacturer.


From looking at 14 CFR part 61, section 61.5 lists the different ratings. While it differentiates sea and land, there is no "ski" category.