tedster - 4:42 am on Jun 6, 2012 (gmt 0)
Well, I stand corrected. At SMX Advanced today, Matt Cutts clarified that these warning messages in Webmaster Tools are manually triggered and a sign of a manual action - as opposed to an algorithmic ranking change. Algorthmic shifts do not trigger a WMT message. See You & A with Matt Cutts [seroundtable.com]
Matt also said that 99% of manual actions will receive such a message in WMT.
Note: Also according to Matt today, Google has dropped using the word "spam" internally and now just talks about manual actions and algorithmic changes.