TechARP claims Microsoft plans to ship Windows Vista SP2 to OEMs in April 2009. A publicly downloadable version should be available sometime later.
A confidential source has revealed that Microsoft will deliver a Release Candidate in February, 2009 with the final SP2 binary scheduled for RTM (Release To Manufacturing) in April, 2009.
As always, do remember that when Microsoft announces that a product is RTM, it doesn't mean you will be able to download it right away. RTM merely means it is "released to manufacturing", including OEMs. Actual public release will be sometime later. Release to web (RTW) will also occur sometime after RTM.