On the Silverlight SDK blog there is a sample that introduces each one of the 11 new easing functions and allows you to change their properties.
Easing functions allow you to apply custom mathematical formulas to your animations. For example, you may want an object to realistically bounce or behave as though it were on a spring. You could use Key-Frame or even From/To/By animations to approximate these effects but it would take a significant amount of work and the animation would be less accurate than using a mathematical formula.
Currently, there are 11 new easing functions that you can apply to your animations. You can apply these using Blend 3 beta or you can use XAML or procedural code.