Researchers from University College Dublin have conducted an analysis of anonymity on Bitcoin, and found it is not inherently anonymous, and that in many cases, users and their transactions can be identified. They use techniques such as context discovery and flow analysis to investigate and visualize an alleged theft of Bitcoins, which, at the time of the theft, had a market value of approximately half a million U.S. dollars.
Read their blog for more (plus nice graphics!)
Bitcoin is not inherently anonymous. It may be possible to conduct transactions is such a way so as to obscure your identity, but, in many cases, users and their transactions can be identified. We have performed an analysis of anonymity in the Bitcoin system and published our results in a preprint on arXiv.