AFAIK, any time money is guaranteed, it can be prorated over the remaining years of the contract. As an example: everyone's favorite Slot Machine received a $5.5M signing bonus in '07. That can be prorated at $1.1M/yr over the 5 years. He's also due to receive a $3.5M option bonus this year, which can be prorated at $875K over the remaining four years of the contract.
Similarly, the Pats could guarantee Brady's salary, and prorate 1/3 of it over each of the next three years ('08, '09, '10).