Driving IT

When looked at objectively, many different sources are used to “drive” changes/updates in IT architecture.

Changes to the RDBMS, e-mail, Network Servers, topology, etc… can alternately be driven by application development, operations, desktop support, PMO, Steering Committee or sometimes even from the desk of the CIO.

My question is, who drives the changes in technology (i.e. Netware vs. NT, MS SQL vs. Oracle, Windows 2000 w/ AD vs Windows .NET) in your organization, and what benefits are there in that method? What problems?