The following changes will be made to the API, effective Monday, Octobert 3, 2009 12:00am EST

• Deprecation of POST/PUT for variant title.
  Clients will no longer be able to modify Variant Title through POST/PUT methods. As Variant Title is a concatenation of Option1 Value, Option2 Value (if applicable), and Option3 Value (if applicable), clients should modify these values instead.
  GET methods for Products and Variants are unaffected, and will still return Variant Title as the concatenation of Option Values for a given Variant.
  

Shopify API documentation:
Products API
Product Variants API