Confirmed: Next 3 Linux Mint Releases Will Be Based On Ubuntu 14.04 LTS http://bit.ly/1v4cOVF
This means that Linux Mint 17, 17.1, 17.2 and 17.3 will all use Ubuntu 14.04 LTS as a base instead of being based on newer Ubuntu releases, allowing the Mint team to "push innovation on Cinnamon, be more active in the development of MATE, better support Mint tools and engage in projects we’ve postponed for years".