At the time the law was passed, Berlusconi was on trial in Milan for his alleged role in bribing a British lawyer for false testimony.
The trial was suspended, pending a review by the court.
If the court determines the law is unconstitutional, the Milan proceedings could resume, raising the prospect of having an elected prime minister on trial.
But Italian media said the court's 15 judges were split down the middle over the case and could delay their verdict by two weeks.
The opposition has said that early elections may be held if the court rules against Berlusconi as he might have to resign under political pressure.
"Nothing will make us betray the mandate that Italians have given us"
Silvio Berlusconi, Italian prime minister
His allies have said they will stand behind him regardless of what the court determines.
"Nothing will make us betray the mandate that Italians have given us," Berlusconi, a billionaire businessman-turned politician, said this week.
Berlusconi has had a history of legal troubles stemming from his private interests, but his supporters say the law was necessary to spare Italy's senior office-holders from politically motivated persecution.
Critics said the law was tailor-made to spare Berlusconi.
As a result of the law, two other cases against Berlusconi have been frozen.
He is accused of tax fraud and false accounting over the acquisition of TV rights by his family-owned broadcaster Mediaset.
Another case involves allegations that he tried to corrupt opposition senators.
Berlusconi denies any wrongdoing.
Prosecutors in those cases have appealed to the constitutional court, arguing that the immunity law violates principles enshrined in the constitution - including that all citizens are equal before the law.