AlyssaS - 1:02 am on Feb 4, 2011 (gmt 0)
Backdraft - if you are in the same spots in the SERPS for your main keywords, and your total traffic is the same, but it's just not converting, it looks like the problem might be medium to long tails? They are ranking you for new long tails where you arn't as relevant, and not ranking you for the old terms?
The only real way to check is take your analytics for today (when the day ends) and compare it to the same day last week, and try to cross reference phrases to see where there has been a drop or rise. It could also be some individual pages have been dropped by Google and they are featuring other less useful pages on your site - the only way to tell is to pore over the stats.