Yes, there WILL be a Book 8!
Here's an exciting (but not unexpected) bit of news:
Diana Gabaldon's OUTLANDER VIII, The eighth volume in the bestselling family saga, to Nita Taublib at Delacorte Press, in a major deal, for publication in Fall 2013, by Russell Galen at Scovil Galen Ghosh Literary Agency (US).
Thanks to SusiQ at LOL for letting us know! The announcement was posted on the publishersmarketplace.com site. It's a pay site, and I don't have access myself, but the summary is available if you google "outlander viii 2013", if you want to see it for yourself.
So, Diana has been saying for months and months that ECHO is not the last book in the series. (With all the cliffhangers at the end? How could it possibly be the last book?) Now we have confirmation from the publisher that there will be a Book 8.
Note that it doesn't say that Book 8 is the last volume in the series, either. I suspect this is because Diana herself doesn't yet know if it's the last book or not! (As she's stated on Compuserve several times since ECHO came out.)
Fall 2013 seems like a long, long way away, but keep in mind what Diana said in her recent blog entry:
The books are invariably longer and more complex than I think they'll be, and _always_ take longer to finish than I think they will.
General observation is that in terms of actual research/writing/etc., it takes me _about_ two to two-and-a-half years to write one of the big novels of the main series. That's baseline.
That "baseline" figure doesn't include the time she spends traveling, making public appearances (such as the recently completed book-tour of the UK), and working on other projects.
So, it looks like Book 8 will follow the pattern of the more recent books, and be released about four years after ECHO.