ArcGIS API for Flex 2.x API
Applications built with ArcGIS API for Flex 2.x requires Adobe Flash Player 10 or above. See http://www.adobe.com/products/flashplayer/systemreqs/ for more details.
To build applications with ArcGIS API for Flex 2.0 (update 2 and later) requires Adobe Flex 4 SDK (final) or later. We recommend that you use Adobe Flash Builder 4.5 as your IDE.
Note that earlier beta's of the API worked with the Flex 4 beta and Flash Builder 4 beta which is no longer available from Adobe.
ArcGIS API for Flex 1.x API
Applications built with ArcGIS API for Flex 1.x requires at least Adobe Flash Player 9.
To build applications with ArcGIS API for Flex 1.x requires Adobe Flex 3 SDK. See http://www.adobe.com/products/flex/systemreqs/ for more details. You may use either Adobe Flex Builder 3 or Adobe Flash Builder 4 to work with the 1.x API, as long as you set the Flex compiler to compile with Flex 3.x.
Using Flex API and ArcGIS Server
The ArcGIS API for Flex, both versions 1.x and 2.x, will work with any REST-enabled ArcGIS Server, i.e., ArcGIS Server 9.3 or later. Certain functionality might require a specific server version, for example 10.0 or above for editing.