I'm reading this article by Greg Shipley. He points to 2001 as the start date for "intrusion-prevention systems (IPS)". This is incorrect, the first IPS was "BlackICE Guard", which we shipped in 1999. It is now sold as the IBM Proventia G.
That year isn't a start date for IPS so much as the start date for Windows-based worms like CodeReda and Nimbda. IPS is good for a lot of things, but it's by far the best technology for dealing with worms. I know of at least three critical financial networks that could not filter CodeRed by port, but which kept up and running because BlackICE Guard could filter the worm but let normal HTTP through.
We had a hard time convincing customers of the value of IPS prior to 2001, after which it was easy.