A thread at WebmasterWorld named Image Results For "your search" reports that Google was messing around with some of its user interface for a few minutes. The member reports that when conducting a normal search at Google, at the top of the page, you would find a line of text that read IMAGE RESULTS FOR "KEYWORD PHRASE". When you click on that link, it would take you to where you would go if you would click on the "image" tab.
GoogleGuy kind of admits it by saying "You know us; we're always testing out different little UI changes.."
