I don't know much about it. It is something I am trying to get more information about. I know a shaykh in my area said it is permissible, but he is too busy to be able to explain why at the moment. He said he will explain later.
But here are some things relating to why it is haram:
http://islamtoday.com/show_detail_section.cfm?q_id=1157&main_cat_id=5
And from the alkuathar student forums:
Assalamualiakum,
As we mentioned in class:
1. Any repayments that are indexed payments, indexed to the Consumer price index etc.. are not allowed by the majority of the scholars and is considered riba.
2. The definitive proof that shows that student loans are in fact riba repackaged is that the indexation is applied annually just like a conventional riba loan. If it was the proper indexation which a minority of the scholars of Islam allowed, then the indexation should only be able at the end - on the day of repayment of the loan, not annually such that the indexation is compounded.
3. The the indexation is applied in inflation, not in deflation and as a result, the indexation is more in resemblance with riba.
Wallahu alim.
Abu Yusuf
http://forums.alkauthar.org/showthread.php?t=1376