Based on the type of software we are talking about, in my opinion IT stuff needs to be involved from the beginning, and throughout all the buying and deploy process. At the end, this time they are the final users, so their opinions about the solution is mandatory. I've also seen at times someone from management making all the decisions and push the final solution onto their IT guys.
At time the choice was good and perfect, but rarely. Many more times the IT stuff feel upset and passed over, and an imposed solution is going to be opposed by them...
Luca.