I bumped into these things accidentally the other day, and am really tempted to order one to see if it works. According to the product description, it allows you to connect both a DVI and a VGA monitor to a properly-equipped DVI video card.
This adapter takes your DVI signal and displays it simlutaneously on a VGA and a DVI monitor. It works as both a DVI to VGA straight-through adapter and a splitter allowing you to use a VGA monitor and a DVI monitor simultaneously.
Note: Your DVI source must be DVI-I and capable of simultaneous digital and analog output. Very few video cards are capable of this.
But for most users, This cable does not mirror an image on two displays. It allows you to have two displays connected to one source (i.e. projector and monitor) and to use one display or the other without having to connect or disconnect the cables which can lead to connector stress damage.
The question is, does a mini have a video out that would support this? Anybody ever tried one of these doodads on a late model Mac? That would be so sweet if you could use one to add a second monitor to a mini.