Traditionally, government financial programs, when cancelled, tend to grandfather current particpants into the program if the law is changed.
A few things: if you and your spouse file seperately, your spouse's income will not need to be applied towards your IRB amount.
10% of elidgibel income is your income minus pre-tax deutions and a predetermined poverty level. For example, if you make 250k and are a single, you substract 150% of the poverty level for a single person (I think its about 15k right now) and pretax deductions (i.e. savings accounts, health spending accounts public transit, etc). Thus on an income of 250, if you have 17k (the max that can go into 403/405 accounts for 2012), 15k for the poverty adjustment, and 3100 for your HSA, you will pay 10% of (250-17-15-3.1)= $21,490.
Now, not all people would have to pay $21,490 a year with a 250k salary. How? When you enter IRB (i.e. day one of residency) the governemnt calculates with interest, how much would it cost for you to pay off all of your loans in 10 years: this amount is the ceiling of your payments. THerefore, even if you income greatly increases, you will pay either 10% of your income of the ceiling amount (you pay which one is lower). For example, you gradudated from a southern med school and left residency with 160k in loans, the total cost to pay the loan back in 10 years would be about 225k (or 22.5k a year). In the above scenario, 10% of your income is $21,490 and your ceiling is 22.5k, so you will pay $21,490. Let's say in two years you are promoted from an instructor to an assistan professor position and your income increased to $285. Now, (285-15-17-3.1)*.1 = 25k. Therefore, for this year, you will pay the ceiling amount of 22.5k instead of 25k.
The only part of IRB I'm not sure of is that the rules were recently changed on whether you pay 15% of 10% of you income. I though it was that any loan that originated on or after July 1, 2011 would be at 10% and any loans before that were at 15% of income, however, the government may have changed the program to make all loans (current and future) at 10%. Hopefully someone can clarify this point.
Edit: I wanted to add some more stuff
If you are thinking of doing IBR, your years in residency count towards your ten. Since pre-tax deductions, you can get creative with system. If you have a spouse that can support both of you, it's worth looking into maximizing your retirement savings. For example, if your salary is 52k, your pover adjustment is 22k (150% of poverty line for two people) and your max out your retirement (17k), your salary of 52-22-17= $1600 a year in loan payments. Once you leave residency, your could roll your retirement accounts into a single IRA and convert it to a Roth