Private markets is one of those features that can really only be appreciated in an enterprise environment. A much requested feature that will be coming in Mango, it will allow companies to distribute vertical apps to their users merely by emailing them a deep link to marketplace.
Apps still however need to be certified (currently only a 2 day process) which should still help guarantee a good user experience, even to the plebs using the vertical apps.
A much more open process is the ability to distribute beta applications which needs no certification and which lives for 90 days in total. Recipients will have to be registered by the developer via their Windows Live ID, so the solution is still not a perfect home brew solution, and only 100 users can be certified at a time, but for reviewers it will be god sent.
The scope of Mango is seemingly increasing day by day, and appears to be addressing many significant user demands. For enterprise in particular there is still quite far to go, but private markets is a good start.