There's actually a useful Zapier script that already does this [0]. I remember reading about it on Mike Knoop's (Zapier cofounder) blog [1]. I haven't tried it out but you should be able to hook it up to any of the alert services that Zapier integrates with. (Which is a lot!)
It uses the awesome HNSearch API to poll for new mentions of phrases and triggers a Zap whenever it finds one. (I have it send me an SMS).
Here's a better, more direct Zap Template: http://zpr.io/gsjS