I doubt Mitel will ever attempt to re-brand ShoreTel sets or add ShoreTel handset support the MiVO 250. The overall look, design philosophy, and call control protocol is 100% foreign. It's a monstrous amount of work just in programming alone, especially due to proprietary call control. If Mitel put any effort in, they would add MiNET support to ShoreTel systems, which an even greater monumental task, and would make little sense. The teams would have to go over the code and learn the programming habits of ShoreTel for the past 20 years and attempt to add a completely new concept.
They did pull this off with the Intl 5000 because they hired basically the entire Inter-Tel dev team. This same team by the way is essentially developing the 3300 code and a lot of the old timers are now retired or gone.
The MiVO 250 added a price point and market segment that Mitel couldn't touch before. If ShoreTel has such a segment, perhaps there will be development interest, otherwise I'm with ace on this one, support for a while, then migration path.