There are signs that Google may have started to implement their anti-spammy site algorithm. At least there are a number of people complaining about a substantial loss in traffic. Where are the people complaining about a big increase in traffic? Where is this traffic going? Are fewer people searching? No word from Google, but they did warn us. Discussion at http://www.webmasterworld.com/google/4258897.htm and a lot of other forums I’m sure.
What can you do? Keep your site clean and keep creating content that people actually want to read. Our sites have only normal fluctuations in traffic. Another suggestion, hire me to help.