OCCalendar is a very simple component for iPhone/iPad that provides a "Popover" date picker controller. It is very easy to add to your project, and is 100% CoreGraphics code, so it uses no images, and is resolution independent. I realize that I need to cut down the size for iPhone a bit more. I originally wrote it just for iPad, and my paths and sizing are all just slightly too wide for the iPhone. I'll fix it when I get a chance.
Via Vincent Demay