My research indicated that requiring a link back -- and that the link NOT be nofollowed -- was by far the most important criteria.
If the "syndicating" site does not attribute the content with links back to the original that are valid for search engines to follow, search engines have a much harder time tracing where the content originated, and must apply complex "find duplicate text content across the whole of the internet" heuristics.
I'm not sure any more than that is necessary.
Related Matt Cutts video
Matt said that it would be a good idea to use rel="canonical" to point back to the page where the article originated - just as he has often suggested that syndicated articles include conventional links (ie. an
<a>nchor tag) pointing back to the original article.
Bear in mind that canonical isn't just slapping
rel="canonical" on an
<a> tag; it's more like this:
<link rel="canonical" href="http://example.com/foo">
So it requires a different sort of work, you have to modify each page header. I'm not sure many of these "syndicators" will have that level of control versus a simple link (sans nofollow!) back to the source.