Various talks and presentations have been given on IDP3 throughout its lifetime.
Here is a copy of a power point presentation given by Dean Hines:
There's also several NICMOS newsletters that mention developments in IDP3 that are also a good source of what the program can do. They can be found at : http://nicmos.as.arizona.edu/software/ IDP3 information starts in Volume 2.
And of course, don't hesitate to read the new and improved help widgets that are on most of the windows in this program!