The current issue of Science has a short article about a 740,000 year old ice wedge found in a section of permafrost in Yukon, Canada [Froese et al., 2008].
An ice wedge is a vertically oriented section of ice the forms in periglacial environments. Water enters a crack in the soil and then freezes, forms an ice wedge, and expands the soil crack further, making the wedge larger (check out this cool animation). It is associated with permafrost.
As the climate warms, there are concerns that melting permafrost will release large amounts of CO2 [Zimov et al., 2006]. However, we don't know exactly how permafrost and related periglacial features will react to warmer temperatures.
The researchers found an ice wedge under a volcanic ash layer, thereby predating the ash layer. The ash layer was dated to over 700,000 years ago. Not only does this make this ice wedge the oldest dated ice in North America, but it shows that ice has survived warmer interglacial periods in the past.