So far as I've seen, there's no way to force the system to provide you a boot option screen by default. Holding down the Option key WILL give you the boot option screen, though. And one thing that gets installed with the drivers is a Control Panel applet that lets you select the default boot OS, so all you have to do is, once XP has loaded, go to the Control Panel applet and choose "OS X" and it will AUTOMATICALLY boot into that, letting you choose XP through the Option key method.