Kevin,
For passive tags, they are getting incredibly small – check this out:
The cost for passive tags can get down near a buck. The variety of active tag technologies is amazing. Overall, it looks a lot more feasible now than just a couple years ago.
For supplies, I would probably not tag a box of 4x4s, but if you use inventory controlled / standardized / wrapped plastic supply bins (speed loaders) to restock your ambulance shelves, that quite feasible and practical. Managing supplies (rather than equipment) fits more in with a supply chain / inventory management strategy than an asset management strategy, but it's the same technology.
A brief article on RFID technology in EMS is going to be in the upcoming issue of EMS Insider.
All the best,
--- Mic