>Intra-entity RI belongs in the Data Tier, Inter-entity RI belongs in the Business Logic Tier.

I like this summary.

>My question is, though, what do you call an "entity" - that is, where do you draw the line? For example, it seems clear that phone numbers are attributes of Employees, i.e. part of the same entity. However, Employees are part of a Company, which may be part of a Conglomerate, which is part of the NYSE, etc. etc.

This is where the imprecision English trips us up.

Employee is not part of the company. The relation is "works at" between the company and employee entity.