Hi Rob.
First I'd like to refer you the Web Services Reference where you can find a description of the RetrieveMultipleMessage and RetrieveMultipleResult classes and their properties.
The typical usage of RetrieveMultiple would be if you already have the IDs of several projects.
However, if you're trying to retrieve information about your active projects (I assume you don't have their IDs in advance), I believe MyWorkItemsQuery or WorkItemsQuery will better suite your needs.
Hope this helps
Ophir