There is a fairly convenient, fully native way to do it, with only a small amount of setup. It's not a single keyboard shortcut, but it is a keyboard shortcut.
Put fast user switching into the menu bar, then enable a keyboard shortcut to focus it.
Go to the Users & Groups pane in System Preferences, then to the Login Options section. Choose to show the fast user switching menu (in any form).
Cmd-drag that new menu item as far left as possible. (This makes it the first to be selected by the keyboard shortcut, but really it can be placed anywhere.)
Go to the Keyboard pane in System Preferences, then to the Keyboard Shortcuts tab. Continue to the Keyboard & Text Input category, and choose a shortcut for the "Move focus to status menus" item.
If you followed step 2, then pressing your keyboard shortcut will focus the fast user switching menu. You open the menu by pressing the down arrow, then select a choice with either
This shortcut lets you directly select a user, so you can choose to bypass the login window or select it from the menu.
I would recommend a shortcut of
control + fn + enter, as this puts your fingers on the return key, and hence just above the arrow keys. Plus, I don't know of any other shortcuts it would collide with.