If you have tried to switch to Design Mode in Adobe Flash Builder 4.5.1 and see nothing but white, look at your Design Mode Problems tab: Window > Show View > Other > Flash Builder folder > Design Mode Problems.
If you see this message:
SWC file failed to load. Any component dependent on this SWC file will not be displayed in the Design Mode.
The SWC may have failed to load because of:
* Incompatible definitions caused by usage of a different SDK version
* Missing referred class definitions
Switching my SDK from 4.5.1 to 4.5 was a suitable workaround. In my case, my custom mobile skins library depends on sdks4.5.0frameworksthemesMobilemobile.swc which didn’t want to play nice with 4.5.1 apparently.