Large law firms should assume that their systems have been compromised by hackers. Whether the danger is state espionage (e.g. Potash hacking of Bay St.), "Anonymous" or cybercriminals in California, firms are either currently being exploited or about to be.
Obviously guarding the front line (i.e. lawyer/assistant computers) is important but generally front line computers aren't the real target. Often the targets of hackers are wire and ACH transfer systems (viz. payroll).