A simple trick for forcing the today screen to re-read from the registry (or refresh) is by sending the message WM_WININICHANGE with the parameter 0x000000F2 to a window called the DesktopExplorerWindow.

Here's a small code snippet on how to accomplish this programmatically:

void RefreshTodayScreen() {
  HWND hWnd = FindWindow(_T("DesktopExplorerWindow"), _T("Desktop"));
  SendMessage(hWnd, WM_WININICHANGE, 0x000000F2, 0);
}

and in managed code...

[DllImport("coredll.dll")]
static extern IntPtr FindWindow(string class_name, string caption);

[DllImport("coredll.dll")]
static extern int SendMessage(IntPtr hWnd, uint Msg, int wParam, int lParam);

const uint WM_WININICHANGE = 0x1a;

void RefreshTodayScreen() {
  IntPtr hWnd = FindWindow("DesktopExplorerWindow", "Desktop");
  SendMessage(hWnd, WM_WININICHANGE, 0x000000F2, 0);
}