Greg Rundlett came to the following conclusion at the EMWCon in Houston:
BlueSpice is really the best option out there - I think globally - for MediaWiki Enterprise, MediaWiki hosting and support. I mean they have put together a real organization. They do a great job of not just assembling the bits and pieces of MediaWiki, and the whole stack; but also Markus Glaser has put together a lot of custom code and extensions to enhance functionality. They have a great system for actually moving content back and forth, using what's called WebDAV technologies, so that you can seamlessly access shared drives as part of your wiki.
Buts that's it. Aside from BlueSpice there really aren't any hosting companies that put together any kind of featureful, current, maintained and customized full stack deployment of MediaWiki."
Except for the fact that there were a few more besides Markus ... I took that gladly to the memory.