Yes. It is just more difficult and time consuming for them than it used to be but they can, and do, find out.
In the past, The Digital Millennium Copyright Act allowed copyright holders to issue subpoenas to Online Service Providers (OSP) seeking the identity of alleged infringers without a judge's consent. This was contested by Verizon in the case RIAA v. Verizon, and as a result copyright holders must first file a "John Doe" lawsuit against an infringer. The OSP is still required to supply the information expeditiously, and if ETBU receives a valid subpoena concerning your file sharing activity, we will be compelled to turn over your identity.
Some content adapted from IU.edu © 2009