Firstly, it is Sondheim and so will probably not sell well to the masses. If EP and MB are both to be in it, it will need to sell massively well just to cover the salaries of EP and MB (neither of whom come cheap!!!!) So probably disaster from a financial point of view, I think.
Secondly, I have to say that I did see EP in this in New York in 2004. Thought she was good in the role (she was nominated for a Drama Desk Award for her performance), but I didn't think she was amazing (like she was in Sunset) and it didn't sell that well over there. At the time, EP was quoted as saying in the US press that, although she enjoyed playing the role for a limited run like she did, it was not a role she would want to play for a more lengthy run.
More recently, Elaine has said that a return to doing 8 shows a week is not something she was considering any more. So all in all, although I would love to see it happen, I venture that it is extremely unlikely. I actually think that MB and EP doing a run in any musical together is much more a sentimental idea, rather than a viable one which will ever come to fruition. Concerts together, yes; an actual fully-staged musical together, not likely.
What would be the chances of them being seen together what with all the 'prior engagements' that will take them away from whatever show they do? If they do!
I think there'll be a large scale revival of Sweeney Todd but not until the present generation of Sweeney boppers can afford to pay for top price tickets ie probably another 20 years.
Even if it is 2011 I'd prefer to see Bryn Terfel and Catherine Zeta Jones in the roles. MB and EP are obvious but uninspired choices.