On Word 2007 to 2013 on Windows, you can use Ctrl+Alt+V.
On Word 2011 for Mac, you can use Control+Command+V.
Both of these will bring up the "Paste Special" dialog. There's no shortcut directly for "Unformatted Text", but since you can use arrows to go to "Unformatted Text" and Enter to confirm, this is probably the fastest way without a macro.