I have heard of this, and I seriously looked into it. However, at least at the hospital I talked to, should for some reason you chose to not fulfill you committment, the interest rate to pay back the money they gave you is outrageous, i.e. I was told 18%. This hospital was a small hospital in what is considered a rural community, so they do not have as wide a range of specialties as in larger cities. I didn't want to make the committment because some of the specialties that I am really interested in are not offered here. If you are interested in a specific hospital, you can contact them and speak with their medical recruiter, or some such similar title. All you can do is ask.
EDIT: Oh yeah, and they prefer you to have been accepted to med school before a contract is signed, so I guess TECHNICALLY its not a pre-med contract, just a pre-practicing physician contract.