property provides a reference to the data object from the DataSource, to which the respective TagCloud item was bound. It does not store the actual data in the ViewState, and that's why it is lost between postbacks. If you want to access the current DataItem, you should bind the TagCloud on every trip to the server, or bind it declaratively in the aspx.
We are planning on introducing an additional Value property for the next release that will come handy in situations where the user wants to associate more information with the respective TagCloud item.
the Telerik team