There will not be a second edition of Nielsen and Chuang.
This comes directly from Michael Nielsen himself, in a comment on his blog. I must say I’m disappointed, because the book of theirs in question, Quantum Computation and Quantum Information, is a fantastic overview of the field… but it’s starting to show its age (published in 2000).
Also, the blog article that this quote was from is quite amusing, as it has to do with sheep and haiku and quantum mechanics. Intrigued? Please check it out! :)