According to Google-focused blog Google Operating System, Google is retiring the Google Web Alerts and replacing it with the comprehensive alert this week. With this change, Google Alerts will no longer be able to monitor when something appears in the top 20 results for a Google Web search only.
It is still a good tool for keeping track of important things, but the Web Alerts will simply be rolled into the comprehensive Everything Alert (which appears to include web, news, blogs, video and groups). Users will still be able to filter results in Google News, Google Blogs, Google Video and Google Groups individually if they choose.
Google Web Alerts No Longer Available [Google Operating System]