goodroi - 4:45 pm on Feb 25, 2013 (gmt 0)
I regularly receive those messages for my websites because my traffic can dramatically change due to seasonality and current events. Google also sends out notices when there are dramatic drops.
The more information we get out of Google is a good thing but I would be careful not to read too much into these messages. For example if you are making several changes to your site, it might not be the most recent change that caused the dramatic change spotted by this notice. Some parts of Google's algo have a delayed reaction.
If this notice is about a dramatic decrease of your site traffic then I would be grateful that Google is helping me realize traffic dropped. Most of my notices the drop was expected since I deal with seasonal traffic swings. Other times it would have helped me to spot content hijacking & competition but I had already spotted the drop myself.
Still a nice reminder to make sure you didn't forget to check analytics.