I just came across this post on the Diesel Ebookstore blog:
To our patrons of the Mobipocket format —
We regret to inform you that Mobipocket pulled all Agency Five titles from all their distribution clients including us. Going forward, the Mobipocket format of all titles from Hachette, HarperCollins, Macmillan, Penguin and Simon & Schuster will no longer be available. We contacted Mobipocket and found out that they currently have no plans to resume A5 distribution in the near or distant future.
Just to be sure, I went and checked Fictionwise. Much to my surprise, Fictionwise doesn't sell Mobipocket format ebooks anymore. I don't mean that they've dropped ebooks form the A5; Fictionwise has dropped the format entirely.
And just so I'd have a third data point, I checked BooksOnBoard. BonB has a really good search function, and I was able to check for ebooks in Mobipocket format by each publisher. There were none.
This really shouldn't surprise anyone. Mobipocket are still operating under the old distributor system where they took a 65% cut of the list price (and shared it with the retailers). The A5 want the split to go the other way, so Mobipocket had to drop the ebooks. It's as simple as that.
Most of Mobipocket's income comes from their role as a distributor. Now that the A5 have made that role irrelevant, I can't see any other outcome beside Mobipocket dying off.