From another thread here, with connections to a newspaper article, it notes that the 747 ADV is proposed for by 2010. It would use the engines being developed for the 7E7 (thus spreading out development costs). The article also notes that the 747ADV would also try to be a 'niche' a/c, between the 777/A340's and the A380, with special long haul sleeper seats, flat beds. This article also noted that production is at 2-3 a month of 747's, there is a modest backlog of orders good for a couple of years and about 2/3'rds of orders are for Freighter versions.
I would also assume that the 747ADV could also incorporate the composite technologies to be used in the 7E7, also spreading out development costs. Todays computer technologies can also help in the ADV designing and engineering, to improve fuel efficiency. There are still many customers who's politics and MX
people are familiar with the 747 and may not want to stray into Airbus a/c's as would have to retrain.
I think we will see 747's being made in some form until 2020.