The origianl plan was for Mesa to start Dash 8 service to replace the service UAL was going to loose by Air Wisconsin phasing out the Do328s. No one has heard much about it since the original announcement. The 328s are going away quickly. I believe Air Wisconsin may only have 3 or 4 left. They are supposed to be completely retired in July.
As a side note... UAL put a gun to the head of its current express carriers and forced everyone to rebid on the United Express contracts. In order to keep express flying, some workgroups have taken concessions so as to be competetive with Mesa.
Everyone (Air Wisconsin, Skywest, ACA
, and MESA) are waiting patiently to see who will be express carriers in the future. We hope UAL will make the announcement soon. Probably within a couple of weeks. I beleive the other companies with competing bids are Trans States and Chautauqua.
So to answer your question... I don't think anyone (even UAL) really knows yet.