That's why it's a good idea to start an account and being posting updates - or "tweets" - in order to establish yourself as the real voice of your firm.
As the Iran election shows, people are getting more and more of their information from Twitter. This trend suggests that many people are looking for information about attorneys on Twitter as well. There's also this attorney's personal anecdote on how he found clients on Twitter and other social networks.
Twitter is currently experimenting with a beta version of a verified accounts service for celebrities and public agencies that authenticates the source of the tweets for users, and lets the account holder feel secure that no one else is sending messages under their name.
Unfortunately, this service isn't currently available for law firms, but I imagine it will be eventually.
Just like with the Great Facebook Land Grab and the availability of Google Profiles, it's important to use these tools provided by companies to help manage your brand's presence on the internet.
In other words, control your message, or risk having someone else control it.
Twitter Launches Verified Accounts (Mashable)
Coming Soon: Verified Twitter Accounts (The Blog Herald)