Skyhook loves to tell you how recent their data is. They re-drive high turnover areas (colleges) a few times a year. Everything else is quite recent. With Skyhook's high profile use in the iPhone they can't afford to be out of date.

Last I checked they had over 45 million access points recorded. With enough users, the system can become self sustaining. You upload all the detected access points in your area for a request so it's trivial for them to auto-update their system if you submitted a newly discovered or changed MAC address.