I’m building a product that will ship after .Net 4.0 is released.
Is MEF an integral part of .Net 4.0, so I can count on them being released together, or is it some external component that might be released separately and thus it is dangerous to use?
Yes, MEF is the part of .NET Framework 4.0. It is located in assembly System.ComponentModel.Composition.dll. You can learn more here: MEF in MSDN