It's Not Just Flash--Apple to Developers: No Silverlight
Apple's Steve Jobs said the main reason you won't be seeing Flash on the iPhone is because Flash is riddled with bugs. Microsoft feels that Silverlight stacks up well against Flash, at least in regards to security issues. Security for the latest release of Silverlight, version 4, has been of prime concern to Microsoft.
Corporate Vice President for the .Net developer platform Scott Guthrie said Silverlight 4.0 is ready for prime time, adding that "Any code written in the last five years tends to be more secure than the code written 10 years ago - that's true for us and others," Guthrie said. "Silverlight, because it's been written over the last two to three years, is able to incorporate the best practices we've learned. We feel pretty good about Silverlight's security."