As a user, I never want applications (or tabs) to take focus unless I specifically requested it. I have gone to great lengths to prevent tabs in my browser (Firefox) from taking focus for this reason.
Jay,
You are seeing designed behavior. To limit opportunities for malicious behavior, scripts running in tabbed windows cannot affect other tabs.
For more information, please see Tabbed Browsing for Developers at http://msdn.microsoft.com/en-us/library/ms537636.aspx :
"The ability to open multiple documents within the same browser window has certain practical and security implications [...] Active tabs (tabs with focus) cannot be affected by scripts that run in inactive or background tabs."
BR.
I'm reasonably certain you can't shift focus to another tab.
My understanding is this is done to somewhat limit pop ups and other malicious content from stealing the users focus.
If the other "tab" is part of your application (and not content from another site) perhaps you should include it in a popup div on top of your main content instead of in a separate window; that way you can always control focusing it, deactivating the content under it (for modal dialogs), hiding it, etc.