Here's a decent summary article about the recent changes.
The formula is to basically figure out what 150% of the poverty line is. Currently $16,775 for a single person living in the lower 48 states (higher in AK and HI). Say your income was $20k for the year. Under IBR, you would not have to pay more than 10%(15% under the old program) of your income over and above 150% of the poverty line. So the total annual payment would be $322.50, no matter what your loan balance is. If your income was $16,775 or lower, you'd have to pay nothing.
If you're married or have dependents, the poverty line number increases. Also, currently IBR doesn't look at your spouse's income. So your spouse could be making $1M/year, and you could still potentially qualify.
In addition, IBR doesn't look at your total taxable income, but rather, your adjusted gross income. So between stashing money into pre-tax 401k/traditional IRA accounts, HSA's, educational savings accounts, and whatever else, you have some wiggle room to lower any potentially required payments.
Further, if your income suddenly goes up one year (I think if you make more than around $60k/year you no longer qualify for the program) so you don't qualify, you could get right back on the program the following year if your income goes back down. So, an ERE type, could potentially preserve a majority of his taxable capital gains as unrealized and then clump them all together in one big year, pay the min loan payments for that year, and then go right back into IBR the following year.
Also, you could arrange your taxable investments to be largely in things like muni bonds, which don't add to your federal AGI. You could be making a million dollars a year off of muni bonds, and still qualify for IBR.
But might IBR disappear like it seems Obamacare might?
I suppose anything's possible, but it seems unlikely to me. It's been around for almost 6 years now and seems to be pretty well established. If they're not going to allow people to discharge student loan debt in bankruptcy, they pretty much have to have some other kind of relief program.
What does seem likely to me are modifications to the program. Particularly to close all the loopholes I just exposed above.
@jacob Some loans given to students for 'living expenses' are at 0% until after graduation, so I suppose there is a small opportunity for some arbitrage there. The real boon though would be taking out $10k-$20k/year for 'living expenses' for like a decade while you get a bachelors and a couple of graduate degrees, living off of part-time work and investing the federal loan money all along, and graduating with $100k-$200k+ invested and with a debt obligation that you don't really have to pay off if you're happy with an income below ~$30k/year. It's kind of unbelievable.