That's a new one. I'm sure there are some examples out there already where bitcoin was used as collateral.
For sure some pawn shops will do it in the future.
The reason, I think, why banks do not like it, because they can't mess around aka rehypothecate with such collateral. After transfering my Bitcoins as collateral to the bank, I can check anytime if it's still there. If not, I could track it.
That's why they also don't like physical as collateral (not as easy to use as the bank's own collateral). A friend told me that he had a very hard time to get a loan when he was offering physical gold coins as collateral. He ended up converting his physical into an ETF paper and that was accepted as collateral.
Definition Rehypothecation:
http://www.investopedia.com/terms/r/rehypothecation.asp