Here's some great Flex resources to help you get started... (thanks to Iris for posting these in the forums!)

Purchase “Programming Flex 2: The comprehensive guide to creating rich media applications with Adobe Flex” by Chafic Kazoun and Joey Lott from Amazon.com. This book helps you understand the Flex framework as a whole and even how it works from a Flash player perspective

Download Flex SDK or buy Flex Builder 2 from Adobe: http://www.adobe.com/products/flex/

Flash Develop: http://www.flashdevelop.org – a great free development environment for creating Flex, Flash, or AIR applications using the free Flex SDK.

Grab the Flex 3 Beta 2 from http://labs.adobe.com/technologies/flex/

Flex 2 Style Explorer:
A great lway to explore the design and skinning of the various Flex components along with the properties and CSS to go along with them.

Flex 2 Component Explorer
If you're more interested in sample MXML and usage rather than styles, check out the component explorer.

Tutorial:
Designing Flex 2 skins with Flash, Photoshop, Fireworks, or Illustrator
http://www.adobe.com/devnet/flex/articles/flex_skins.html

Lots more Flex components (with source code and samples!) that weren't necessarily developed by Adobe:
http://www.flexbox.mrinalwadhwa.com/

The official developer's guide from Adobe. I use this all the time to look up specific components and how to use them (along with any Actionscript questions)

For the latest Flex news and thoughts from Ted Patrick, Adobe's Flex Evangelist, visit http://www.onflex.org/

A large poster-sized PDF of the Actionscript 3 API

A large poster-sized PDF of the Flex 2 API

Adobe Integrated Runtime (AIR) for JavaScript Developers Pocket Guide free download