Matt Cutts talked about it in response to a submitted question in a recent Webmaster Help video:
He said, "It's clear from the way that people are talking about it that there are a lot of low-quality guest blogger sites, and there's a lot of low-quality guest blogging going on. And anytime people are automating that or abusing that or really trying to make a bunch of link without really doing the sort of hard work that really earns links on the basis of merit or because they're editorial, then it's safe to assume that Google will take a closer look at that."
"I wouldn't recommend that you make it your only way of gathering links," Cutts added. "I wouldn't recommend that you send out thousands of blast emails offering to guest blog. I wouldn't recommend that you guest blog with the same article on two different blogs. I wouldn't recommend that you take one article and spin it lots of times. There's definitely a lot of abuse and growing spam that we see in the guest blogging space, so regardless of the spam technique that people are using from month to month, we're always looking at things that are starting to be more and more abused, and we're always willing to respond to that and take the appropriate action to make sure that users get the best set of search results."
But you already knew that, right?