A few week ago I discovered Postgres Schemas (worst named feature ever -- nothing to do with SQL schema definitions). Basically, schemas allow you to have multiple databases within a single database. So, you only need to manage (or host/pay for) a single database but each company can have their own schema which gives you complete separation of data across all the tables.

These, together with the Apartment gem for Rails, makes building isolated multi-tenant applications really easy. It also makes migrated to a true multi-database/multi-server/sharded setup much simpler.