Why is the @ character ignored in search queries?

Matt's answer:

CUTTS: Atul Arora from Fremont, California asks, “Any reason why Google search does not treat the @ sign differently given the rise of Twitter? For example @mattcuts and mattcutts give me the same results.” Well, historically, that was a deliberate choice because we didn’t want an index email addresses, at least I think it was a deliberate choice. You don’t want somebody scraping Google to find a bunch of email addresses. So it’s kind of nice not to index the @ sign. It depends. I could imagine this over time starting to treat that differently, but at least for the time being that hasn’t been a popular or a request that we’ve heard enough that we would probably put resources into it right now.

by Matt Cutts - Google's Head of Search Quality Team

Original video: