Well, there are some books that are not released in electronic format -- the Harry Potter books, for example -- but I haven't noticed Amazon (or the publisher) delaying release of the electronic version when a new book comes out in hardback and there are already electronic copies available of the earlier books in the series. Now, if there's a change in publisher, there could certainly be a change in policy.
For example:
Phantom Prey by John Sandford was released in
hardback on 8 May 2008, and
the Kindle edition on 14 May.
The Cold Minds by Kristin Landon was released
for the Kindle on 24 June, the same day that
the paperback edition was released. This book's initial printing was in paperback -- there was no hardback release.
Where I get frustrated is that
The Cold Minds is second in a series and the first book isn't available for the Kindle. Yes, I already read it in paperback, but I gave my copy away and I'd like to get the Kindle version for keeping and re-reading, but will have to wait until *someday* when the publisher gets around to dealing with the back-catalog.