The absolute earliest you'll see any Sandy Bridge chips at all is 2010. But Intel has stated repeatedly that it will update architecture every 2 years. Meaning Sandy Bridge will debut 2 years from the debut of Nehalem. That means late 2010, possibly even Jan 2011.
Just because Intel says 2010 for Sandy Bridge doesn't mean you'll see it in 2010. It's somewhat dependent on when the previous architecture debuts. If there are production hangups, problems with the design, etc. that pushes it back further.