I understand the desire to stop message storms, but in my 5 minute use case, I hit that and perceived it as a limitation and it makes it probably not usable for me, as I already have a good monitoring/notification/paging system that I was trying yours out as an additional notification vector. (That might place me squarely out of your target market, in which case you should ignore this feedback. :) )

Let's say I already have an existing monitoring service that sends emails on network events. I might decide to hook in your service and send my network faults there.

I get "port XX on switch YY link state down" and that gets routed through the system. One minute later I get "office of the CEO video conference network down" but pagerduty never sends that to my mobile device because I'm busy looking at the one port down alert.

Yes, I realize there's a technical solution to create multiple pagerduty trigger email addresses, but at a minimum, I'd encourage you to be more clear about that feature/limitation.

Overall comments: several UI elements were "not pretty" looking on IE7, and some of the call to action graphics "Create your first alert now" were bright yellow and rectangular yet not clickable. No deal-breakers, and I was certainly able to quickly get setup and trial alerts coming.