There are several different forms that multi-tenancy can take.
The simplest for Mendix is to use multiple mendix instances, on different ports, for different customers. This is effectively multi-tenancy of the hosting hardware platform (and database, perhaps), but not the application. Each customer then has their own licensing, could have their own theme, etc.
True Multi-tenancy within a single instance of an application is more complicated. The application needs to be designed with this aim from the ground up, so that all data is segmented by permission groups, including any configuration data and permissions to manage that data. It can be done, and we (Buoyant Solutions) have done this for our e-ServiceSuite application See blog here. Also see listing in the App Store.
Other challenges for true application multi-tenancy include theming for different customers (which can be done through alternative start pages), and more problematically, managing the allocation of licenses between different customers, which currently cannot be done easily.
The first example, with multiple instances, is often the best solution when there are separate companies involved, but the second example is more suitable where different parts of the same organization have separate workflow needs, or data privacy requirements.
We - Mansystems - are developing a multi tenancy solution in our ExpertDesk product. Using instance access and a bit of modeling it is possible.