You probably have heard this: SQL Server 2008 is RTM per Aug 6. It means it is released, and it is available. A supported product.
So, if you're going to build something from the green field, I would suggest SQL Server 2008. I know not everyone is in the "cutting edge" area of the technology adoption curve, many plays conservative and have the n-1 syndrome (I'll post more on this later), but it is a good idea to have a look, especially if you're a developer building data-driven app.