Bad News first
Even after a rehab, the late or other deragatory marks will remain. It will usually be only back to the date that the loans were transferred to the company you were working with for the rehab.
For example: I had a different batch of SL that I rehabbed last year. It showed my last derog as the month before the loan was transferred to the company that let me rehab. So the derog was already aged 1 year (which is good).
Rehabilitation DOES NOT make the credit history (derogatory, because otherwise you wouldn't be in rehab) go away. I've heard of mistaken reporting where it does, but this is an annomily (sp?)
Good News
A Defaulted Student loan is a nuclear bomb to getting credit. I think, in a way, it might be worse than a BK. With a BK, you don't have to pay back the money owed and it eventually goes off your CR. A student loan will NEVER go away if unpaid.
So with a rehab, the Defaulted Student Loan notation will be removed and you'll have your last derog note about a year aged.
Hope that helped.
P.S. don't even get me started on the SPECIAL rules that do not allow Student Loans to be discharged in BK. If I lose 10 million in a start-up biz, I can get it discharged in BK if things go south. I invest 100K in education and lose my job and can't pay SLs = no BK. Gosh I wonder, if Big business was behind that legislation...