Tony Schreiner, a developer of Internet Explorer, talks about the development of IE7 and the use of tabs:
Our philosophy for tabbed browsing is to keep the user in control of the experience. Tabs are on by default in IE7 Beta 1, but for those that do not want to use them they shouldn't be intrusive and there's a setting to turn them off and reclaim the screen real estate if desired.
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. The rationale for opening only customized windows in a new frame is that this seems to correlate with scenarios where showing a window on top of the current window is desirable, such as replying to posts on internet message boards and getting a close-up view of items on shopping sites. Naturally we will continue to refine the default behavior and provide settings to customize the behavior.
In addition there is ongoing work to tweak the DOM so that window operations behave in a compatible but non-intrusive way. For example, window.move() will continue to allow a web site or applications that automate IE (more on this later) to move the frame, when there is a single tab open, but when multiple tabs are opened this will have no effect. These tweaks will allow IE to behave as expected for compatibility, but should help prevent sites from tampering with the browser when you have a set of tabs open.
IE7 Beta 1 has most of the core features you expect from a tabbed browser. You are able to open links in a new tab by middle-clicking or Ctrl-clicking links. You can switch between tabs quickly and easily using both the keyboard and mouse. You can control whether tabs open in the background or foreground, or open them in a new window as you always have. This core functionality is largely catch-up to other browsers which support tabs, but a necessary foundation for future work.