Can/will Unity 3.0 and Unity 2.6 be installed on Windows and both be usable without clashing?

This question is intended to be for the Pro versions, but applies as well to the standard Unity version. Can a Unity developer have both versions installed and successfully build apps with both on the same machine?

Yes. We (Unity Devs & QA) usually have many many different versions of Unity installed at the same time.

There aren't really any Pro "versions" of Unity...Unity Free and Unity Pro are the exact same app. The only difference is your license code. It's possible to install and work with different versions of Unity on the same machine now, so I wouldn't expect that to change.

Yeah, you can even have multiple copies of the same version. Unity Editor doesn't require any registry settings, nor does it install or reference files in any other directory but its own, so its xcopy deployable. For our project, we source control the editor.