A guy over at prusadjs has hacked up his old MSI Wind 3 cell battery, attached a USB connector and now it is capable of charging an iPhone.
The iPhone takes about 2.5 hours to fully charge and the netbook battery is left with about 50% of it's charge left meaning a second full charge of the iPhone should be possible.
The main downside of all this is the big temperature increase in the battery due to the 7805 regulator that reached temperatures of up to 120 C according to the guy hacking it up. He thinks with a better design you can bring that temperature right down though. The other downside is that the battery is unusable in the netbook once opened and tampered with, so make sure you have another spare if you intend using your netbook after doing this.