(In the UK) IT came first and made sense because the services were "technology" oriented - supplying and maintaining the hardware was the core job, even software development was seen as supporting the hardware.

In the 80's there was a huge shift where departments wanted to change how they were perceived and broaden the support and services they provided beyond the core hardware.

So, "IT" became viewed as old and sedentary, while "IS" was new, active and forward-looking.

It was a marketeers wet dream, so IT departments up and down the country got their name changed for no good reason other than the board wanted to be seen as being hip and with it (horrible, like your dad dancing at the school disco)

So, IT->Hardware, IS->Value Added Services. Your internal department is more IT, an out-sourcing company is more IS.

Nowadays most people don't know the difference so the terms have become interchangeable.