Freak number of referrals triggers a penalty?

Large referral spike caused my traffic to plummit

[Wi11]

Looking at my traffic stats I've spotted some weird Google behaviour. Here are the details:

Google traffic to my site was averaging around 4-600 searches a day.

Then I put up a news story about a new very popular widget that had been launched. I must have been one of the first sites to put up this story so had a top three ranking for about a day. I got a relatively large traffic spike with my total of searches going to 800ish for that day.

Straight after that spike went down my Google traffic is now between 100-200 searches a day. Looking through my stats it looks like that sudden activity has caused a Google penalty to kick-in? :(