In the movie "Zeitgeist", Peter Joseph shows a zodiac chart, focuses in on the middle of it, where it resembles a cross, and says that the cross is "shorthand" of the symbol.
First of all, he's specifically using a post-Christian example of the zodiac chart which happens to have a cross in the middle of it. If you go to google images and type in "zodiac chart", you'll get many examples of zodiac charts. You'll also notice that most of them don't have a 2-line intersecting cross in the center, but either have an empty circle, a 6-line intersecting cross (creating 12 sections), or some other image in the center. In other words, Joseph cherry-picks a chart which corresponds to what he wants to claim. Had the symbol for Jesus been just about anything else, he probably could have found a zodiac chart which corresponded to that.
Also, there's no evidence that followers of the zodiac used whatever image was in the center of the zodiac chart as "shorthand" for the chart itself. I don't know where Joseph got the idea that the cross was shorthand for the zodiac chart, but it's not from anything that dates to ancient times.
It should also be noted that the pre-Christian zodiac didn't always have twelve sections. The Babylonian zodiac had 18 signs, and the Mayan zodiac had 20.