Is there an 'Open Folder...' keyboard shortcut in VS Code?

There is indeed a shortcut for it, the command is workbench.action.files.openFolder, and the default keybinding is Ctrl+K,Ctrl+O.

It's easy to find in the list of shortcuts (File → Preferences → Keyboard Shortcuts) if you search for "folder" or "openFolder" in the Default Keybindings.

Hit Cmd+O on macOS or Ctrl+O on Windows/Linux to activate the open dialog, select a folder, and click "Open."

Note: This answer was written before multiple folder workspaces were implemented in VS Code. If you open folders this way, it will replace your entire workspace with the folder you selected to open. If you want to add a folder to your existing workspace, you must use Cmd+K, Cmd+O on a macOS or Ctrl+K, Ctrl+O on Windows/Linux.