Well, we have them in a formally informal way. We require that they post their home contact info, log into the internal chat server, and keep a log or include what they worked on while home in their monthly status report.
Work at home is just that-work. We do not allow WFH for a sick kid, car trouble, etc, as in those situations, not much work will get done. However, if you need to be home to sign for a delivery, or the furnace guy is showing up for service, that is fine.
We do not allow scheduled days from home, except for those folks whose job can be done more than 90% of time online. There are not many of those positions left, since we offshored them.
We also do not allow work at home during critical periods, such as end of quarter closings. Also, if you are meeting with anyone outside of the dept (i work in IT), then you need to be present.
The guys on my staff would rather be able to WFH when needed as opposed to scheduling a regular day anyway, and having them onsite is better for business.