The only thing I have learned is that even almost finished titles have a tendency to be postponed. That means that some time schedules if made is just pure guesswork. Take this title for examplehttp://books.global-investor.com/books/14333/Nassim-Taleb/Static-and-Dynamic-Hedging,-2nd-Edition/Due for publication on 16 Jan 2011 - can be pre-ordered

It have had an ISBN number for ages, of course. That is searchable and the internet book stores usually list the books, that only have an ISBN, but is not yet published. Now if Paul said, that there will be a third edition in X months time; he'd tank his own sales to some degree wont he, and that mail campaign would be for nought. So, not registering an ISBN would make sense also.