An interesting approach to job alerts was released today by TheLadders group of jobsboards. In basic terms, it allows an employer to specifically choose to send job alerts only to candidates who have chosen to follow that company. It seems to be taking a cue from the Twitter model in doing so, and allows companies to open a "first selection" intake of applications only from candidates who have expressed a specific interest in working for them. The model allows the alerts to be sent out at a later stage to everyone else (i.e. filtered at a sector/location/salary level instead) if the company did not find what they were looking for the first time round.
An interesting concept in that it promotes the idea of "preferred employer" lists for candidates, and a potential ready pool of candidates who (should) have already bought into the company a bit - and ideally may have taken time to research a bit about the company in advance. Of course it leaves itself open to candidates signing up to every company in site, so I would think it would only work as a model if a candidate had a maximum limit that they could subscribe to.