You have your nifty new hand-held ultra mobile PC running Windows Vista, but you wish your font size was a little larger so you could read the text. You hunt around and find that you can modify just the font size (from the classic view of Control Panel -> Personalization -> Adjust font size (DPI)), so you select "Larger Scale (120 DPI)" and click OK hoping to be able to read your text in larger font. You reboot the system but when it comes back the fonts are exactly the same size as before.
You may wonder "What happened?"
Here's the issue. When you make such a request to Windows (to increase or customize DPI), the first thing Windows does is check your screen height. If your screen height is greater than or equal to 600 pixels, it will do what you ask. But if your screen height is less than 600 pixels, it will totally ignore your request. The reason for it is the boxes and dialogs and even the start menu with the larger font won't fit on your display and may make it near impossible for you to close a dialog (because both the top bar and the OK/Cancel buttons are off the screen at the top and bottom). They could have added some sort of warning, but since Vista didn't support lower than 600 x 800 and there weren't many screens at lower than 600 pixels screen height at the time of development, this did not seem necessary.
So now you know. There is even a KB for this. KB969397