I'd like to offer a few thoughts about the unofficial timeline and see what others think.
Avaya says that they are looking at the end of 2010 for the Partner's retirement (about 4 years from now).
From what people who are familiar with the One-X have posted, the system does not expand as much as the ACS. There are a few operational quirks mentioned as well.
My W.A.G. is that Avaya chose 2010 thinking that the 4 year cushion will give the company enough time to tweak the One-X to the point where it could replace a Partner ACS system of
any size.
If Avaya has indeed released many of the employees that worked with traditional telephone systems, the chances of seeing a new traditional telephone system to replace the Partner ACS are slim to none.
