##Gravitational Constant
In your description you mention the gravitational constant is 0.5 for performance reasons, in the code it is actually 0.1.
##This Pointer
There are times when the this pointer is necessary, but it is not necessary in the code under review (EntryPoint.cs, Body.cs and BodySystem.cs).
##Variable Names
To some extent this is personal preference, but the 2 letter variable names are generally not descriptive enough. I can see x and y as positions, but I don't necessarily understand dt (deltaTime), Vx, Vy (?velocity?), Ax, Ay (?acceleration?), G (?gravitational constant?). Variable names should be descriptive so that comments aren't necessary.
##Comments The param comments for name, mass, foregroundColor and backgroundColor aren't necessary, the other param comments would not be necessary if better variable names were used. Keep in mind the more comments there are the more the text in the source code needs to change when maintenance is performed. By using self documenting code and only commenting important algorithm information, edits to comments are kept to a minimum.
##Private Versus Public
There is at least one function in Body (float DistanceBetween(Body otherBody)) that could be private rather than public.