Found via Slashdot. Following my previous post about IE7 tabs, more detail information about this new feature has been revealed by Tony Schreiner, one of IE development team member. A quote from the post:

“Regarding script, there is no “target=’_tab’” feature or any direct access to tabs from script beyond what is available with multiple windows today. We are working on balancing the default behavior for whether a window opened from script opens as in a new frame or a tab. Currently, windows that have been customized, such as hiding a toolbar or making the window non-resizable, will default to opening in their own standalone frame, whereas ordinary pop-up windows will open in a new foreground tab. CTRL-clicking and middle-clicking links will open those links in a background tab.”