
If you press and hold Command-Shift-Space Bar, a HUD-style box appears at the center of the screeen and lists available input keyboards. Release and press the Space Bar while holding down Shift-Command cycles through the available choices.
[robg adds: For US users, the default Input Source shortcuts (Command-Space and Command-Option-Space) conflict with Spotlight. I was able to see the HUD panel only after reassigning the Input Source shortcuts; I set them to Option-Space and Shift-Option-Space. The screnshot at right is from my machine; click it for a larger version.]

