I have been to Milano and I know that there are more Juve tifosi than either Milan or Inter. I went to see Inter v Juve in 2000 (we won 2-1) and there were a lot of Juve fans from Milano. Lombardia has more Juve fans than Piemonte albeit by a small margin.
Although the population of Torino is around 1 million while that of Milano is around 1.4 million I don't think that it affects the attendance to the stadium. While in Torino there are more Torino fans (I think it's like 40-45% Juve and 55-60% Torino) there are still a lot of fans in the area to fill the stadium and you must not forget that thousands of fans come from all around Italy to watch them play. Juve has some 10 million supporters in all of Italy.
The problems that I know of is that the Delle Alpi is far from the city centre like the old Comunale used to be. This makes it less accessible. I really think that they should have restructured the Comunale instead of the Delle Alpi but maybe they didn't do that cause they wouldn't have had enough free space to build Mondo Juve (the complex with shops, restaurants, theatres, etc).
The capacity of the new stadium should be between 35,000 and 40,000 as far as I know which is too small in my opinion but I also heard that it would be easy to add another ring to the existing and increase the capacity of the ground like Man Utd did. So if the stadium is always full and there is continuosly more demand I think the stadium could be expanded easily to seat 50,000 - 60,000.
I think it should be ready around 2006/2007 but I am not sure about this. Anyone has more specific details about this?