I cannot believe that this subject is so popular. I also cannot believe that so many people are so paranoid of an sRGB output. I guess I am glad to see a lot of interest in printing and I think both Apple books and Blurb.com are a fantastic way to showcase your work - far better than just jamming them up on the web.
Let me get to the point - I have made blurb books from Aperture and I am pretty certain based on my experience with a lot of digital and traditional printing that there will be no difference with an sRGB output vs a CMYK output. A reader asked if I had hard evidence. The answer to that is no but I would be willing to shed a couple of bucks if there really are a bunch of people interested in hard evidence between an Aperture sRGB/Booksmart sRGB workflow and a full-blown Adobe InDesign CMYK workflow using the blurb.com supplied CMYK/HP Indigo profile.
I happen to be handy with both and since I am planning a new book about my local neighborhood area sooner or later I will be happy to conduct a head to head comparison if there are people that would benefit from this.
Here is the deal - leave a comment on this post if you want to see a head to head comparison. If there are more than 10 people interested I will spend the time and money to do a cage match of sorts. Do me a favor and let me know what books/subject matter you are planning with what type of medium as well so I can try to cover the bases in terms of workflow/color management/image types and associated colors.