Keyboard: F7
Command: View.ToggleDesigner
Versions: 2008,2010
Published: 10/19/2010
Code: vstipEnv0044
You can use F7 to go from the current Source or Design view:
To Code view:
To go back just hit F7 again. It's a toggle so it will jump you back and forth between the views.
Interestingly, in WPF applications, you can use F7 to view code but not to go back to Design view so it isn't a full toggle operation in that particular case.