If you're going to trust someone to give you a pre-indexed version of the blockchain, you are trusting them about the entire history of transactions. They can claim whatever they want (within certain bounds), and your client will accept it. If there is a mistake in the data (either deliberately or accidentally), it will result in the client rejecting the future valid chain at some point, potentially landing you in a side chain (which may be under control of the attacker). If many people would use the same source for their pre-indexed files, they would all end up on the same side chain, living in their own happy world. Both chains (the real one, and the side one) would allow every existing output to be spent once; if this happens on a large scale, it is a disaster for Bitcoin.

Of course, the long waiting times are a real problem (the upcoming 0.8 version should be significantly faster, though) but if you can't spend the resources and time to maintain a fully validating node (like the reference client implements), consider using a lightweight client like Multibit. It does not offer the same security, but it is better than trusting a centralized instance to tell you history.