GovExec has a piece on this subject by an IBM type.
By the nature of our government (weak executive, decentralized, federal system) we're fated to build such systems incrementally and from the ground up. For example, the National Finance Center in New Orleans is one of the providers of shared services. Back in the day (i.e., 1968 when I joined ASCS) the agency had several ADP (automated data processing, for you whippersnappers) centers. I assume they were initial steps in the process of using computers to support operations. Over time, ASCS closed some centers and consolidated in New Orleans and Kansas City. Over the same time, other USDA agencies were going through the same process, leading finally to USDA taking over the NODPC. So it came to support Federal personnel salaries and benefits for the whole department, and then to provide similar services for other units of the government.
In a way the process reminds me of the way our planetary system evolved, as I understand it, by the gradual accretion of material.
Because this is a slow process I get very envious of Estonia (as I've previously blogged) which apparently was able to do a top-down implementation. To use another metaphor, it's rather the difference between a city like Rome, with an ancient history, and a city like Reston, planned and implemented from scratch within one man's lifetime.