It used to be Ctrl+= in Mathematica 7, but in Mathematica 8 instead of writing below this shortcut starts free-form input. The documentation at tutorial/KeyboardShortcutListing hasn't been updated. Which shortcut is now used for writing below?
Below is Ctrl+4 (or $) now. You can find that information in the file
($InstallationDirectory)\SystemFiles\FrontEnd\TextResources\Windows\KeyEventTranslations.tr
(expand $InstallationDirectory in Mathematica to find this directory)