Previous SDKs

I have no idea how long you'll be able to download these for, but currently, you can still get Adobe Flex sdks by updating the url below:

4.5.1:

http://download.macromedia.com/pub/flex/sdk/flex_sdk_4.5.1.zip

(this works also: http://fpdownload.adobe.com/pub/flex/sdk/builds/flex4.5/flex_sdk_4.5.1.21328.zip)

If you want to change it to 4.6, just do this:

http://download.macromedia.com/pub/flex/sdk/flex_sdk_4.6.zip

If you want 3.x, just do the same exact thing:

http://download.macromedia.com/pub/flex/sdk/flex_sdk_3.5.zip

or 

http://download.macromedia.com/pub/flex/sdk/flex_sdk_3.6.zip

There probably is no way of knowing what is available, but until sourceforge gets fixed, this works...hope that helps someone looking for an older sdk.