Gee, I started this thread before I went to bed yesterday night, and now I get back to it to find so many replies. Thanks everybody for pitching in!
Just to clarify, I didn't get that email. However, since I run a top rated backlinking service, I like to keep a close eye on any trends and developments, to find patterns that might help me and my clients grow and protect our businesses.
One way I do this is by setting up Google Alerts based on various topics of interest. This is how I noticed that the number of queries for "detected unnatural links" grew exponentially in the last couple of months.
The number of results for the query [site:google.com "detected unnatural links"] was about 50 when I started following this topic. Now I can see over 300 results. This happened in less then 2 months...
If I extrapolate and think more about how many people would post about this issue within Google's Webmaster Central, compared with how many people would not say anything at all, I think that multiplying this figure by a factor of 100 would be safe, assuming that this behavior is similar with that of posting product reviews on Amazon - only 1 out of 100 people would post a review, sometimes even less.
Anyway, this means that at least 30,000 people got this email. We can play with the numbers anyway we want, but the trend is very clear: Google is up onto something heavy.
I have never used GA, or registered my sites with Google Webmaster. I find though that blaming such notices solely on GA might be too simplistic.
I also don't agree that relying heavily on a single type of backlinks might trigger this notice. This behavior would be too easy to simulate by a malevolent competitor, so Google will never punish a site owner for having solely one single type of backlink. It might discount those links, but not send a Unnatural Links Notice (ULN - ha, I always wanted to create an abbreviation that nobody understands
I really don't care for creating (more) panic so my intention is to investigate this issue thoroughly, as I always do. I have a couple of my VAs already browsing the Webmaster Central, trying to identifying patterns, and I'll keep you posted on my findings.