@misc{cryptoeprint:2008:480,
author = {Daniel J. Bernstein and Tien-Ren Chen and Chen-Mou Cheng and Tanja Lange and Bo-Yin Yang},
title = {ECM on Graphics Cards},
howpublished = {Cryptology ePrint Archive, Report 2008/480},
year = {2008},
note = {\url{http://eprint.iacr.org/}},
}
You will need the url.sty package (comes with most LaTeX installations) for the last line. Otherwise, remove the \url command.