Business Idea: Craigslist monitoring website

I want to be able to monitor craigslist for specific keywords. I want it to be web-based, super-easy signup, notify me of matches via Twitter, email, etc.

Monitize it by showing google ads for whatever they are searching on, or by including ads in the notification emails.

The only ones I can find are desktop applications, which is the wrong way to build this. My sister wants the same thing.  She uses a mac, which is another reason not to build a desktop application.

UPDATE: Ian Drake explains why monitoring Craigslist is difficult.  CL blocks the IP of any server that sends many automated queries.  Ian’s company, NotifyWire, gets around this by having each user run a desktop application, these applications join together to form a cloud that can query Craigslist frequently from many different IPs.