Twitter does, however, intend to roll out a possible solution to impersonation accounts on its site (a violation of its terms of service) with verified accounts. According to the Twitter blog:
We do recognize an opportunity to improve Twitter user experience and clear up confusion beyond simply removing impersonation accounts once alerted. We'll be experimenting with a beta preview of what we're calling Verified Accounts this summer.
Verified Accounts will feature a special seal.
The experiment will begin with public officials, public agencies, famous artists, athletes, and other well-known individuals at risk of impersonation. We hope to verify more accounts in the future but due to the resources required, verification will begin only with a small set.
San Francisco Mayor Gavin Newsom's official site links to his Twitter account.
Please note that this doesn't mean accounts without a verification seal are fake -- the vast majority of Twitter accounts are not impersonators. Another way to determine authenticity is to check the official website of the person for a link back to their Twitter account.