In my experience so far, which is limited to EJB 1.1 and is only a few weeks old, an entity bean is tied to a single table. However, custom finder methods can be defined that use joins, a technique more standardized and easier in EJB 2.0, to find one or a collection of your bean instances based on criteria in multiple tables. The documentation for your J2EE server should include some information on declaring custom finders that use joins.
This thought has just occurred to me based on your post, Kyle: Can an entity bean be based on a view, either via CMP or BMP, EJB 1.1 or 2.0? I have a little work here that takes precedence, but I'll check this thread later and have a look at the relevant specs.