<p>Does Microsoft force an update down to all of its users of windows-update?  Is it for legal reasons?</p>

<p>EDIT(aku):  </p>

<p>Question was reformulated. if you want to know which version of Windows comes with .NET see this topic:</p>

<p><a href="https://stackoverflow.com/questions/71390/which-operating-systems-come-with-net">https://stackoverflow.com/questions/71390/which-operating-systems-come-with-net</a></p>

Yes, Windows Vista shipped with .NET Framework 3.0. I'm sure future versions of Windows will ship with whichever is the most recent version of the framework.
Scott Hanselman recently blogged about the status of .NET 3.5 on Windows Update:
http://www.hanselman.com/blog/UpdateOnNETFramework35SP1AndWindowsUpdate.aspx