This is actually a great question. Robusto's answer is perfect, but there are other times when you'll hear speakers using that manner or "style" of speech where it doesn't fit the scenario described.
For example, say you walk up to a newsstand and ask for a newspaper. The clerk doesn't respond and just goes back to playing a game on his phone. You might stand there for a bit in disbelief, then say, "So, were you going to get me that newspaper?"
All native speakers recognize that use of the past tense for an event that hasn't occurred. In this case, it carries the message that you think it should have been done already. You are conveying irritation, but in a grownup, I'm-making-a-conscious-effort-to-be-civil kind of way, and, possibly, sprinkled with a little uncertainty as in, "Is there something here I'm missing?".