I'm in the process of writing a security report on Heroku. I was interested to know the number of times that attacks on the service caused downtime. I wrote a script to collect all of the 815 incidents and then checked each report against the following expression: /hack|attack|ddos|malicious|virus|vunerability|breach/.
This returned the following incidents: 156, 157, 245, 308, 489, 539, 665, 709, 738.
Discarding false positives gave this list:
In summary, all attacks that resulted in a service outage were caused by DDoS attacks, and all of these were in 2011.
The data can be downloaded here.