Here's the definitive fact on whether IMS will dominate. A colleague of mine works for a large telco, and he mentioned to his management that he'd like to comment about this article either here or in his own blog. The answer from his mangement: he shouldn't discuss IMS in blogs.
So it's laughable, in a sad sort of way, to see the telcos sink billions of dollars into technology when they have no idea of how to use it. Blogs are scarey, unpredictable, and unmanaged (if you work in the sheltered telco environment); but so is the rest of the innovation on the Internet. If the telcos control innovation on IMS, then innovation will happen elsewhere, and given the fact that there's competition from other sources of network connectivity, IMS will rapidly sink into obscurity.
We've seen this before. The telcos introduced X.25 with tremendous fanfare. Restricted entry, defined usage, central control, and billions of dollars of investment backed by the authority of no less than the United Nations. Along came the Internet, which disaggregated authority and ownewship of the network — a powerful combination that allowed a group of underfunded academic researchers to utterly defeat X.25.