This is worth a read:
http://msdn.microsoft.com/en-us/library/bb822049.aspx
Specifically:
"Windows Vista does not support the standalone installation of the .NET Framework version 2.0 SP2 or version 3.0 SP2. Windows 2000 does not support the .NET Framework version 3.5 SP 1, nor the standalone installation of the .NET Framework version 3.0 SP 2."
Also - "If your application relies on changes that shipped with .NET Framework 2.0 SP 1 or SP 2, then you can have your application target the .NET Framework 2.0 and ask your customers to download the .NET Framework 2.0 SP 1 or SP 2. However, if your application relies on new or changed functionality, it is recommended that your application target the .NET Framework 3.5 or the .NET Framework Client Profile. "
Which does suggest there may be a standalone release at some point in the future, though your guess is as good as mine.
Also see this comment on Somasegar's blog from around the time of the beta about the background to 2.0 SP2:
http://blogs.msdn.com/somasegar/archive/2008/05/12/visual-studio-2008-and-net-fx-3-5-sp1-beta-available-now.aspx#8515404
You could unzip the dotnetfx35.exe and try and muddle your way through installing the .NET 2.0 SP2 bits on a throwaway/test machine to see if it works. The files are under wcu/dotNetFramework/dotNetfx20.
But my caveat would be wait until the official RTM or deploy the whole of 3.5SP1, you may, as Scott correctly points out, end up with an unstable and wholey unsupportable installation (specifically MS PSS).