>>396
LockWindowUpdateを使ってみたらどうだ？

[DllImport("user32.dll")]
static internal extern int LockWindowUpdate(IntPtr hWnd);

Cursor preCursor = Cursor.Current;
Cursor.Current = Cursors.WaitCursor;
LockWindowUpdate(Handle);
try
{
//フォームを表示
}
finally
{
LockWindowUpdate(IntPtr.Zero);
Cursor.Current = preCursor;
}