The Share Alike clause is indeed a murky issue. And you're right that it doesn't help matters that "noncommercial" is vaguely defined. We're grappling with these issues ourselves at TechSoup.

If it helps, the rules of thumb seems to be:

• If you're going to create a derivative of a single work that was released under a Share Alike license, be prepared to release your derivative under the same license.
• If you want to create a derivative from multiple "Share Alike" works, then those works should all have the same Share Alike license -- otherwise, don't do it.

Here's the exact info from the Creative Commons FAQ:

Can I combine two different Creative Commons licensed works? Can I combine a Creative Commons licensed work with another non-CC licensed work?

Generally yes; you can combine one Creative Commons licensed work with another Creative Commons licensed work or with another work.

The one big caveat is for Creative Commons licenses that contain the ShareAlike license element (ie. Attribution-ShareAlike, Attribution-NonCommercial-ShareAlike). These licenses require derivative works (ie. the result of two combined works) to be licensed under the same license elements. So, you cannot, for example, combine an Attribution-ShareAlike license with an Attribution-NonCommercial-ShareAlike. If you are combining a work licensed under a ShareAlike license condition, you need to make sure that you are happy and able to license the resulting work under the same license conditions as the original work.

I don't know if this necessarily means that Share Alike is a bad thing. After all, it allows you to ensure that your goodwill continues down the "derivative chain," so to speak. But maybe, in a future version of the license, it would help if Share Alike works could be mixed by "downgrading" to the most restrictive license in the bunch? In other words, mixing an NC-SA work with a SA work could yield another NC-SA work? Just a thought.

The Creative Commons site also has a chart that shows what licenses you can use on your derivative work based on the license of the original work (start with the "row" for the original work and then check the column for the new work):
http://wiki.creativecommons.org/FAQ#If_I_use_a_Creative_Commons-licensed...

TechSoup, NetSquared, the Public Library of Science, and Creative Commons have actually come together recently to start a Creative Commons Publishers Association, which meets just before each monthly Creative Commons Salon in San Francisco. The goal is to try to get clarity on these exact issuese. Even if you're not in SF, I'd encourage you to take part remotely. Check out the wiki and add your questions and comments.
http://ccpa.wikispaces.com

Hope that helps!

--Amit

Amit Asaravala, Mgr. of Editorial & Content Strategy, TechSoup