Ravi Nagarajan has a good writeup on The Real Reason Behind Apple’s Decision on iPad Flash Support (applied to iPhone/iPod, too.) From the article:
Technologies like Flash and Silverlight allow developers to create rich content that can be delivered via a web browser. There is no gatekeeper regarding who may run such content as long as the user has a web browser that supports the plug-in technology. This open access model directly threatens Apple’s obsession with retaining tight control over what applications are allowed to run on the iPad.
Apple is so reluctant to allow users to control the software that runs on the iPad that the device even lacks such basic interfaces as a USB port. Failing to support Flash and other plug-in technologies is presented to the public as an attempt to shield users from buggy software that could cause the iPad to crash. Yet Apple seems content with supporting Flash technology for Macintosh computer systems. If Flash is so buggy, perhaps Apple should eliminate support on the Macintosh computer lines as well?