With respect to the AMA deal, IMO you should always get permission from the land owner to use the land for RC use. Without permission you might be trespassing and at the very least you are performing an activity not approved by the land owner. If the feces hit the fan and the land owner is sued because they allowed unsafe RC activity on their land, you can be sure the first words out of their lawyers mouth will be to the effect of "My client never approved nor condoned RC activity on their land, nor were they aware is was being used in such a manner." If it is a public area owned by government, it is a bit more tricky as some parks may allow this use, others may not. If it get ugly, more than likely the park will hang you out to dry and make RC officially illegal to avoid future problems. Remember, just because it is a public park doesn't mean you can use it however you like. The good news is that planes which would be typically flown in a public park, private parking lot, etc., have a small potential for serious damage because of their typical small weight/speed/power.

In short, have fun, but be safe and take reasonable precautions and you will be fine.