The only thing I could find from apple:
Safari on iPad and Safari on iPhone do not have resizable windows. In Safari on iPhone and iPad, the window size is set to the size of the screen (minus Safari user interface controls), and cannot be changed by the user. To move around a webpage, the user changes the zoom level and position of the viewport as they double tap or pinch to zoom in or out, or by touching and dragging to pan the page. As a user changes the zoom level and position of the viewport they are doing so within a viewable content area of fixed size (that is, the window). This means that webpage elements that have their position "fixed" to the viewport can end up outside the viewable content area, offscreen.
I understand the "works on the iPhone" part...but maybe it doesn't anymore? This could be a change in OS/mobile Safari since the latest public iPhone OS release shipped (the above documentation is from March 2010).
I'm going to re-tag this question adding iPhone to it, maybe one of the guys with the developer 4.0 OS release can test this? If it is the case it's been removed, this should be a bug filed/fixed before it goes live...I'm not sure on how the procedures are on this with Apple are though.