As far as I know, there is no fixed rule about .NET versions, and there is no way to find out which version to expect on a client machine.
The minimal version number to expect can be the one that was installed with Windows by default: None for Windows XP, 2 for Windows Vista, and 3.5 for Windows 7. But these are optional components which can be uninstalled.
As a recent example, take .NET 4:
.NET Framework 4 Client Profile is released as a recommended update on Windows Vista and Windows 7 (automatically installed depending on the Automatic Update settings of the computer). Also released as an optional update for Windows XP, so one has to manually run Windows Update and select it, and the same goes for Windows Server 2003, Windows Server 2008 and Windows Server 2008 R2.
Conclusion: There are no rules, and the results vary for each .NET version and then again for each Windows flavor. Better not look for any persistent logic in the decisions of the Windows Update team at Microsoft, and expect anything and nothing on the client computer.
1Not quite true, I thought that there were a few that would be automatically done if you had your windows update set to auto. For instance, the 3.5sp1 update was automatic to anyone who had installed 2 or higher – Alex – 2011-06-29T00:34:05.350
@Alex: That's not the behavior I've seen before. I will of course nuke this if I am incorrect but I'm basing this off what I've seen real machines do – Billy ONeal – 2011-06-29T00:35:39.057
Go ahead and nuke it. – Hans Passant – 2011-06-29T01:04:40.450
@Hans: Okay, I would have been happier with a reference with more proof, but as two have already commented against it I changed it. – Billy ONeal – 2011-06-29T01:25:23.977
@Alex @Hans: do you have a reference? – Casebash – 2011-06-29T03:54:32.337
@Billy - don't nuke it - Microsoft Update DOES update .NET frameworks if you have them installed. – Rory Alsop – 2011-07-08T09:50:04.300