I use the Shift + F7 often to switch between source and design view.
Does anyone know of a hotkey to switch between the source file
Yep just F7
Look at this poster for C# VS2008 shortcuts
You can also bind it to any combination of keys you like, go to tools, options (show all settings) environment, keybord, Show Commands for View.ViewCode and change to your liking.
Added by Rob Cooper:
For 2005 Users: Visual C# 2005 Keyboard Shortcut Reference Poster
For 2010 Users: Visual C# 2010 Keyboard Shortcut Reference Poster
Edit 08 Sept 2014
There don't seem to be posters beyond 2010 but there is a website for VS2012, VS2013 and VS14: Visual Studio Shortcuts