Bloomberg reports that GE agreed to power the -800 and is in talks about putting engines on the -900 as well. They definitely won't power the -1000 as this plane is in direct competition to the 777-300, where the GE90 is the exclusive engine. Plus, the GE90 cannot be used on the A350-1000 as there is an exclusive agreement with Boeing in place.
To me, this is a rather bad solution. Why the heck is PW not jumping on the train??? They wanted to supply engines for the 787, so I'm sure they would still be capable of doing so for the A350. This way they could regain market share big time.