There's no secret setting in Dell's customized launcher to change this, but as you can see in the photo, there is a way to get the home screen and everything on it to rotate into portrait mode when you turn the Streak upright. It doesn't involve rooting or flashing a custom ROM either, so anyone can do it.
All you need to do is head over to Android Market and install ADW.Launcher. It replaces Dell's launcher (excluding the status bar) with an incredibly customizable one that looks a lot like Android 2.2. In addition to the auto-rotation feature, ADW lets you determine the app grid (number of columns and rows) you want in portrait and landscape modes and has settings for removing the app labels, hiding the status bar, tweaking the UI, adjusting desktop scrolling and overshoot speeds, and applying themes.
Another nice thing about using this launcher on the Streak is that you don't have to crop your wallpapers to a non-standard resolution. 960 x 800 parallax-sliding wallpapers that correctly rotate along with the home screen are a go!
The difference a new launcher can make is pretty astounding. The Streak almost feels like a different device!