MDL Handbook webpage
Awhile in the making, Andy Kopra, Jan Jordan, et al. have written a handbook for the Material Definition Language.
For those of you not familiar with MDL, you can find the main page describing it here.
Any software, such as mental ray 3.13 and Iray will be able to exchange materials using MDL and render identically. Software integrating mental ray, Iray, or MDL itself will also be able to use and render these materials, Allegorithmic is a noted partner.
Take a look at the MDL Handbook page for more information on using, creating, and integrating MDL.
You can now find the presentation online here: MDL Presentation