Comment by userbinator

Win32 standard multiline edit controls use Ctrl+Enter to insert a newline (instead of pushing the default button or "submit" action on a dialog), so that may be where the idiom came from.

For me, Enter to send and Ctrl+Enter for newline is the norm in an IM application, while longer and more asynchronous communication (like this textbox on HN for commenting, or a forum post, or an email client) implies that Enter inserts a newline and something more substantial (Alt+S is common, or Tab,Enter to move to and press the submit button) submits.