Now that ESRI has officially released ArcGIS 10 there is both the need and desire to download and use the latest and greatest Flex API and Template. The ESRI Flex 2.0 API is based on Flex 4 which is most readily available in Adobe’s new Flash Builder 4 (formally Flex Builder 3). It is possible, and very doable, to bring Adobe’s Flex 4 sdk in to Flex Builder 3 but it is extremely easy to import a Flash Builder 4 project into Flex Builder 3.