They are referring to bonding. Your panel should have neutral and ground but they should not tie together in the panel. You will have one ground strip and one neutral strip. In your main panel they will be tied together with a buss bar
This is correct. Even though you may have multiple sub-panels, the Neutral and the Ground wires are only connected back at the main panel by a bar. The main panel ground is connected to an earth ground, which is a long rod into the ground or sometimes a conducting water pipe that goes under ground. This point is located near the meter. Ironically, the power into the house only had three wires, two hots and a neutral. Some place out in the power system there is an earth ground connected to the neutral.
So if you are running new wire definitly run a 4 wire system so that you can connect the ground an neutral to gether back at the main, not at the garage. On the other hand if the garage was a long way from the main (200 ft.?) and you already have good three wire feed I would not hesitate to sink another ground rod and create an earth ground at the garage. Yes, this may cause some electrical noise, but I believe it would be safe.
Somewhere at some point I seem to recall reading in the many codes that if the distance to the sub-panel that is in an external building is over some distance away from the main then a second earth ground should/could be used.
By the way, while there is a national electrical code it is not a fixed law or something. Each city or other governing body can rewrite the code to their specifics desires. It is common that they will take the NEC (National Electrical Code) Handbook or the NFPA 70 National Electrical Code and just adopt sections of it, not the whole thing. NFPA (National Fire Protection Association) is not a law by the federal or state governments. It is a private organization that writes and promotes standards. Likewise, even though it has the word National in it I think that NEC handbooks are written by a few. I have the 2008 version a well as other versions but have quit purchasing them as they are a money making effort by the organizations. ~$150 for the NEC handbook. My version was authored by 4 individuals who I am sure make money doing so. They change it essentially every year and I believe that it is used to promote new products by industry. My making some things part of their code the companies can then charge more and the old products, where the patents have expired, are no longer the standards. If you do not believe me, just go to you hardware store and look at the differences in the prices of a simple electrical outlet plug. The older devices cost $0.50 while the newer still patented devices typically cost $3-$4. As an example, electrical outlets that are "child proof" are now suppose to be used according to the Code. They have a plastic inserts built into them that only lets one plug in a device if both prongs are inserted at the same time. However, they tend to break and they do not hold the inserted plug well. So the circuit commonly fails due to this poor mechanical system.
Likewise, there is something called "spark arrestor" or "arc fault circuit breaker". It has taken the ground fault interrupter an additional step. It is also required by the code .... in some cities. In my town they are required in bed rooms. It is suppose to monitor for arcs such as from a frayed lamp cord etc. This is ok, but plug in a vaccum cleaner, which uses a brushed motor and the brush arc will trip them. A pain!
