Here's a recent report by the ECB itself: http://www.ecb.int/pub/pdf/other/virtualcurrencyschemes20121...

Here's an analysis of that report by economic analyst Tuur Demeester: http://www.zerohedge.com/news/2012-11-04/bitcoin-seen-throug...

Here's an interview of Tuur Demeester by Max Keiser about his report: http://www.youtube.com/watch?v=d5vowdygIPU#t=12m42s

Long story short: The ECB is worried about Bitcoin because unlike e-gold and other centrally clearing solutions, the peer-to-peer bitcoin doesn't have a central guy doing all the transaction clearing so there is no-one to bully and arrest.