Absolute Positioning & ViewportFor the desktop web, the vast majority of content is developed in flow layout (as opposed to absolutely positioned). Historically, this has allowed designers to be relatively indifferent to the pixel resolution of the user's screen. That strategy doesn't work out very well on a mobile device because the resolution difference between the iPhone/iPad/G1/Droid etc. are dramatic and the enviornment in which the device is being used requires tighter control over the position of elements. For example, if a user has to hit a button with only one hand to work the device, the button really needs to be in exactly the right spot (as opposed to off the screen).
Viewport is a new meta tag that is available and is particularly useful for mobile situations.
meta name="viewport" content="width=320, user-scalable=no"
Using viewport, you can design to one resolution (say 768x1024) and have the whole page display on every device you are targeting. In conjunction with viewport, if you use absolute positioning, you can guarantee the fidelity of the user experience.
Cache is King
Animation Is In!
What used to be the province of Flash is now available to the iPhone/iPad/Android/Nokia/BBY6.0 and all webkit based devices. You can move, rotate and flip elements with the greatest of ease. We have one sample that allows you to roll a soda can on a table top - that animation updates 100 times per second and it works beautifully even down to Gen1 iPod Touches. Check out the source code here: http://appMobi.com/Gallery.