Although I was already familiar with DI/IOC his presentation managed to sharpen my understanding of why Ioc is important.
I don't have Mike's exact definition of Ioc but in my words:
"Ioc is where code surrenders control or configuration to external code"
This is very common in most Frameworks including the the .NET framework.
Consider the TrueForAll method from List
So in my opinion Ioc is a very important concept that every developer should understand to effectively implement that Single Responsibility Principle.