Exactly as predicted by TechARP, Microsoft today supplied all its beta testers with a Release Candidate version of the Windows Vista Service Pack 2 and Windows Server 2008 SP2. Microsoft did not mention when that version might be made publicly available, but if all predicitons turn out to be right, we can have the final second service pack for Windows Vista and Windows Server 2008 around April or May this year.
"We continuously engage with our partners and testers on the development of service packs and we will track customer and partner feedback from the beta program before setting a final date for distribution," Microsoft said in a statement. "We will have more to share on Windows Vista and Windows Server 2008 SP2 soon and are targeting final delivery in (the second quarter of) 2009."
At the same time, Windows 7 is also gearing toward the release candidate stage. There's a rumour saying that the next iteration of Windows could reach RC stage at the end of this month, more specifically on February 27, 2008. This will be the only RC build of Windows 7 released, after that the manufacturer will be pushing the official and final product to the market.

Source: CNET News