Hi Cords and Aash,
I came down that way in late June. There are two routs south from Potosi (up to there it's all tarmac). One seems to be the better route, as at the turn off onto Routa 1 near G Bush and Belen the road is signposted as a diversion and gets a little rough. However, Routa 1 was about 250km of roadworks in a quite advanced state. They even had about 40km of tarmac down and the rest seemed to be pretty much ready for tarmac. There a loads of lorries and some spectacular countryside. Closer to Villazon you cross the mountains on a good dirt road. Routa 1 is joined by the other road (sorry, don't have the number) which is also dirt at the turn off. Routa 1 took us two days with a stop over in Carrera (very small place, but friendly people and a cheap room, no breakfast). There are a few nice places to stop over. It took us two days cause Kate was very unhappy on any kind of dirt, and we had two flat tires in two days (third one as soon as we got into Argentina). Don't you love carpenters on road works? Anyhow, I figure they should just about be finished with most of that road. There is a petrol station just before you hit Iscayachi and I think there was one in Camrgo.
Just be carefull about 10km from Villazon we hit a huge sandpit at 50kph just after sundown...That woke us up!
Gareth and Esther went North on the other road and were not happy at all. Check out hteir blog. Link on my blog.
However, I don't understand how you consider the Salar 'not essential'! It was certainly a big highlight for me. The place is just unbelievable, and the round trip Potosi, Uyuni, Potosi is ok, about 4 hrs for 230km with loads of photo stops. I actually did the round trip from Sucre...