All in all, I think there are no real signs that gold is in a bubble at the moment. With real yields around zero out to the 5-year point, gold (probably through an ETF like GLD) is a defensible investment.
One metric that shows gold is in a nascent bubble is the effect that investment demand has had on the metal, without which gold would be in the 800s. Still, even at these prices, there isn't any sign of an all-out bubble. I still believe there will be one, although not in the near future. A ramp-up in inflation would provide the catalyst, because the gold story has spread far and wide enough for an inflation ramp-up to provide a major impetus to piling into gold.