What was the most important data center development of 2009?
There have been significant advances in how we cool data centers in the industry with utilization of higher efficiency chillers, CRAC EC fans w/ variable speed motors, economizers to tap into free cooling and utilization of scrubbed outside air in certain environments have allowed companies to achieve PUE ratings in the 1.15 - 1.2 range.
From a server perspective servers have been increasing in processing power per watt year over year and the wider acceptance of virtualization has allowed the industry greater utilization of processing resources with fewer servers.

What single advance would most positively impact the data center sector in 2010?
I think that the cloud will make a tremendous impact on data centers. Anytime that you can pool resources together into a larger unit to share between users you can do a lot more with less. Storage makes up a small pieces of this because storage in itself is not the most power hungry resource in a data center. As we look forward into cloud computing where we can share processing and memory resources into a large pool for users to pull from then we look to make the greatest gains.