How accurate is polling?

So...I see this morning on FOX and CNN that Obama now has a slight lead in the "poll's", who exactly is polled and what percentage of the voting public does this include. I never saw anyone walking up and down the embly lines asking for the input of the workers, I never saw anyone in Iraq asking the enlisted personnel who they supported. As a matter of fact other than one phone call last year asking me what party I support no one has asked me anything. So it seems to me the polls are not really a very good indicator of how a candidate stands. If it is delegates only, I would think by now they are pretty well decided.