Is this intentional (e.g. the new one revisits same design principles for same target market), or merely a marketing mishap?
The model introduced in the year 2000 is referred to as the PowerShot S100 Digital ELPH in North America, but the Digital IXUS in Europe and most of Asia, and finally the IXY Digital in Japan. The model introduced in the year 2011 is referred to as the Powershot S100 and as far as I can tell this name is kept throughout the world.
It is possible to consider that a certain amount of nostalgia is built into this model, as the model introduced in 2000 was the first Digital ELPH camera when it was released, and 11 years later they have reused the North America model name in the new S100. I do not believe that this model name was used for any specific reasoning, such as to indicate some type of anniversary model or flagship reincarnation, rather I believe it is due to the simple progression of the "S" series of cameras naming convention. The S series evolved out of the S10 and S20 from 1999-2000. With the exception of the S45 and S95, each successive model number has increased by a factor of 10(S30,S40,etc). They simply kept with the model and named the newest camera S100. It is possible on the current path, that in a few years we could even see the S200 name reused.