Comment #4 on issue 2206 by asmeurer: plus-minus object http://code.google.com/p/sympy/issues/detail?id=2206
When you say x = Symbol('x', real=True), you are saying essentially "let x be an arbitrary element of the set R". This is exactly the same thing with pm, except it is an arbitrary element of the set {-1, 1}.
Now, since the latter is a finite set, we can expand it out to all possibilities, but as just pm, it is no different from a Symbol. The only special thing here is mp, which is equal to negative of whatever pm is.
In fact, if we ever get our new assumptions in, we might be able to extend the idea to a Symbol that is assumed to be an arbitrary element of any set that can be symbolically represented by the system (finite or infinite).
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