Why Ethernet? It's for more than infrastructure wiring these days. Metro and Carrier Ethernet connections now offer the best value for the money in terms of dollars per Mbps bandwidth. In many cases these savings come from dealing directly with a competitive network service provider who provides the fiber optic connection.
How far you want to go with this depends on how heavily you want to be involved with the voice and data infrastructure and services. When building or rehabing an office building in a metropolitan area, a good option is a Fast Ethernet or Gigabit Ethernet service and the structured wiring, switches and outlets to distribute the service to individual offices. This also makes sense when developing a new office or industrial park. If fiber optic cables are pulled to each location, along with power and telephone lines, the cost is fairly incremental.
What service level should be provided? That's determined by how many clients you have and their bandwidth requirements. Most every business will want at least T1 level Internet service (1.5 Mbps). More sophisticated clients who need to transmit medical images, engineering drawings, or video can really benefit by Fast Ethernet at 100 Mbps. Some might actually be persuaded to choose your facilities over others if they can have easy and affordable access to Gigabit Ethernet service at 1,000 Mbps.
Another decision is whether to contract with the carrier yourself and become the service provider to your clients, or simply provide the physical infrastructure so that carriers can easily hook-up clients directly. To get a carrier interested, you might at least have to get commitments for a certain level of bandwidth from your tenants. If you buy bandwidth from an Ethernet carrier and then route it to interested clients, you can determine the monthly service pricing and perhaps turn a handsome profit as well as having satisfied customers.
Of course, all of this can be done with traditional T1 lines and DS3 service. But bringing in an Ethernet Wide Area Network connection can slash your costs by half or more if you can use 10, 100, or 1,000 Mbps of bandwidth for your facility. If you are planning or currently providing such services, then find out what carriers are nearby and what the service levels cost in your area.