Before the Mango update, each application could have the search button do whatever it wanted. Mango changed that, and now it will always take you to Bing search. There is nothing you can do to change it back.
edit for comment
What I think you saw was Bing Vision. When you press the search button you should be getting a screen that looks similar to the image below. Notice the red circle. That button will take you to Bing Vision search which will allow you to scan barcodes, QR codes, movies, etc....
Perhaps you had been bumping this button by mistake. As far as I know. No carrier/phone combination overrides the default search button. Every time you press it you will be taken to the Bing search page.