Guys, is it possible to set a property of more than one object at a time?

With Python, yes. Without it, it might be difficult, since you’d have to deal with messages and such. Monster might have made a resource for this, but I don’t quite remember.

SolarLune, please clarify, “In blender I cant select 20 planes (like the leaves of a tree) and change physics to no collision?” r u saying it is impossible with the default installation?

You can if those leaves are separate objects. But I suggest you to keep the whole tree as one object and make another object(simpler) as collision envelope.

In Blender:

add the property to one object
Shift select the others and choose <space> copy game properties

use a Python controller.

What is the purpose of this request?

I created trees, load of it. I just decided that with short ones my character should be able to walk thru the leaves. unfortunately I already have them set to static. I want to change the property of the leaves all at once…

An easier way than to actually duplicate or instance the tree is to place it elsewhere in the blend file and create a group of it. Then, place group instances of the trees - this way, all the trees are the same.

So, is the Problem just editing the Trees to be NoCollision? (Which »Property« would be a bad chosen Term for.)
While SolarLune’s Method is actually better for you (except for when all your Trees look different), you can select all your Trees and change the active one (the One selected with a brighter Selection Outline), then you click in the 3D-View: Object > Game > Copy Physics Properties

… uh, even Blender itself calls them Properties? Well, whatever.^^