Your state may vary, but "Farm Truck" tags in Texas are very limited. You are only legal when working on the farm or:
Doctor
Grocery Store
Church
School
That being said, I've seen some brand new Suburbans with FT tags that I know darn well have nothing to do with farm business. Don't know if the owners are bald-faced liars, smart operators, or what.
http://www.txdps.state.tx.us/lw/cmvlaw/registration/Farm.asp
