The reply via SMS feature is excellent and something that I always want in any monitoring system.
I'm curious about the security around the email alerts, can anyone send to the specific trigger-alarm@mysite.pagerduty.com or can you add at least a 'FROM:' check?
You probably have this on your road-map, but if a ticketing/worklog system could be integrated with this, you could add an incredible amount of value for folks working on the problem in real time.