this comes per an email that a friend of mine sent me:
Title XXIII, Chapter 320
320.39 Reciprocal agreements for nonresident exemption.--
(1) The Department of Highway Safety and Motor Vehicles may negotiate and consummate with the proper authorities of the several states of the United States or any foreign country reciprocal agreements whereby residents of such other states or foreign country operating motor vehicles properly licensed and registered in their respective states or foreign country may have such privileges and exemption in the operation of their motor vehicles in this state as residents of this state whose vehicles are properly registered in this state may have and enjoy in the operation of their motor vehicles in such other states or foreign country. However, nothing herein may be construed to relieve any motor vehicle owner or operator from complying with and abiding by all other applicable laws and rules relating to safety of operation of motor vehicles and the preservation of the highways of this state. In the making of such reciprocal agreements, such departments shall have due regard for the benefits to be achieved for and the convenience of motor vehicle owners and the citizens of this state.
(2) The Department of Highway Safety and Motor Vehicles is authorized to continue membership in the International Registration Plan, a reciprocal agreement among the states and the provinces of Canada which provides for proportional payment of license fees.
(3) No such reciprocal agreement consummated by the Department of Highway Safety and Motor Vehicles may become effective until approved by the Governor; and all such reciprocal agreements shall be made subject to cancellation at any time by the Legislature.
(4) The Department of Highway Safety and Motor Vehicles shall give proper publicity to the terms of every such reciprocal agreement.
All we care about is (1). According to 320.39 (1), if you are legal in your state, you are legal in Florida, in as such that you are in Florida on a temporary basis (i.e. vacation, business travel, etc.) and not traveling to Florida to become a permanent resident.
Ok, for you fellow Alabama riders here is our statute:
Section 32-6-51
Rear tags required.
Every motor vehicle operator who operates a motor vehicle upon any city street or other public highway of or in this state shall at all times keep attached and plainly visible on the rear end of such motor vehicle a license tag or license plate as prescribed and furnished by the Department of Revenue at the time the owner or operator purchases his license.
Anyone violating the provisions of this section shall be guilty of a misdemeanor and shall, upon conviction, be punished by fine not exceeding $500.00 and, in addition thereto, shall be prohibited from driving a motor vehicle in Alabama for a period of not less than 60 days nor more than six months.
(Acts 1935, No. 512, p. 1100; Acts 1936-37, Ex. Sess., No. 31, p. 28; Code 1940, T. 36, §75; Acts 1961, Ex. Sess., No. 143, p. 2085, §§1, 3.)
Nowhere does it state the tag must be horizontal. In Alabama according to the statute, vertical tags are legal. Actually, according to the way it is written, upside down is legal. The section concerning tags is Alabama Admin Code Section 32 Chapter 6, Article 2, Division 1, Division 1. (Yes that is Division 1 twice).
posted to help other's "get all learned up" before traveling...
for ya'
i guess i needed a couple more cups of 
