Google is replacing alerts for web search results with comprehensive alerts that include results from Google Web Search, Google News and Blog Search.
Marcel Gordon says that web alerts weren't very popular, "We've decided to retire Web alerts because they are used by very few people and an alert of type Everything will find the same results. This week we'll be changing all alerts of type Web into alerts of type Everything. You may receive more results after this change. If you find that you are getting too many results, you can change the "How often" setting to "once a day" or "once a week." You can also change your search query. You can do this on the alerts management page, or by removing the alert using the link at the bottom of each alert email and creating it again with different settings."