I have recently purchased a Dell Laptop with a Sigmatel STAC9200 Codec and Azalia High Definition Audio
. The laptop has only one headphone line out. Is it possible to reconfigure this line-out so that it sends out an un-amplified signal?

First, I am not even sure there is a small amplifier circuit after the codec. And if there is one, its distortion when driving the input impedance of a normal amplifier would be minimal.
Second, any mod implies opening your laptop and dealing with surface mount components, with good chances of destroying your sound interface or the whole notebook.
So, all in all, I advise you against it.
Conclusion: leave your headphone output alone and just use it as a line-out.

If you need an attenuator, just solder a couple of resistors on a jack.

You could get an external sound card - USB to line out. Some are quite small.