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I use the Spyder 3 with my HP dv6t Quad Edition. The laptop may not be as good as an external monitor, but it works. I can see a definite improvement after calibration, and I don't have trouble matching my display with my Costco poster prints.
I think using the calibrator with multiple monitors will depend on whether the graphics card supports multiple monitor calibrations. I've calibrated the laptop screen alone, and also the HP IPS monitor that I usually have the laptop connected to. The laptop loads the correct profile when I'm using one screen or the other, but I never use both screens at once.