The blockchain has a lot of information about all bitcoin transactions.
I've heard a lot of conjecture about how anonymous bitcoins transactions are, but I haven't seen a scientific study.
Are there any case studies that show how to track a bitcoin transaction to an individual person? I see IP addresses on blockchain.info so that information could be subpoenaed; on the other hand, you could use Tor which also has its documented weaknesses.
Yes, the most notable one being this one. It illustrates that it is possible to associate transactions and addresses with forum users' identities, as well as connecting them to some organizations (including WikiLeaks, MyBitcoin, Slush's Pool etc).
Tracking users by their IP is a bit harder, although during the work on my master thesis I was able to find an IP address associated with a Bitcoin Address and trace it correctly to the Bitcoin Faucet.
Another interesting point of views, by Dorit Ron and Prof Adi Shamir (- The S from the RSA algorithm) :
https://eprint.iacr.org/2012/584.pdf
From the abstract: "...We downloaded the full history of this scheme, and analyzed many statistical properties of its associated transaction graph. In this paper we answer for the first time a variety of interesting questions about the typical behavior of users, how they acquire and how they spend their bitcoins, the balance of bitcoins they keep in their accounts, and how they move bitcoins between their various accounts in order to better protect their privacy. In addition, we isolated all the large transactions in the system, and discovered that almost all of them are closely related to a single large transaction that took place in November 2010, even though the associated users apparently tried to hide this fact with many strange looking long chains and fork-merge structures in the transaction graph."
