Some service bureaus merely accept .STL files and print them for you. Others will offer tuning capability to ensure they successfully print. Some others offer pre-made models where users merely select and customize. Ponoko connects the right designers to happy clients.
ProTocuBe has yet another approach. They offer a more deep lifecycle, where they will consult (or even create) your design right from the beginning stages and follow through right to production of models. ProTocuBe:
creates digital models and engineers them, according to the customer’s needs and focusing on what kind of product has to be made.
They leverage their 3D technology to produce models, renderings, videos, presentations and websites, typically focusing on (but not limited to) the architectural market. In other words, they provide a consulting service to carry the client's ideas through design and production stages.
If 3D tech is hard for many clients, there certainly are many ways around that barrier.