My wife and I subscribe to this the theory that different diets are better for different people, and it has some relationship to blood types. I don't think it's as simple as "this type eats this food" but I definitely think there is something to it.
I am type O and definitely do best on the Paleo type diet. Type O's were supposedly the "hunter gatherers" so this fits the theory. My wife is type A and definitely does best on a grain and vegetable based diet. Her diet is not Paleo at all and she would not do well with it, she can't eat that much meat and fat and needs her starches and carbs.
We often ask friends and family what their blood types and dietary preferences are and the data points we get are way too common and correlated to be coincidence. Almost without exception, the type O's that we know are enthusiastic meat eaters, while the type A's and B's are either omnivorous or prefer various starches and grains much more than the type O's. This is all anecdotal but I think there is something to it.
This isn't to say that everyone we talk to fits into 2 or 3 diets, it is just a tendency. For example we ask people, if your plate had meat, salad and pasta/rice on it, what would your first bite be? For me, it is always the meat, in fact sometimes I will finish the meat before going on to anything else. My wife does the same with the starch. The answer that people have to this question says something about their general dietary preferences.
I think the science behind books like "Eat Right 4 Your Type" is a little iffy at best, and maybe it doesn't have to do with blood type at all, but I definitely think some other aspect of human lineage that is itself correlated with blood type. Doesn't it make sense that, among the billions of people in the world, people of different genetic backgrounds need different diets based on how they evolved over the preceding thousands of years? There are studies that show that humans evolve in periods as short a 100 years, so over 10,000 years or 300-400 generations there is something ample time for a culture to the theorybecome adapted to a particular diet.
This would also explain why some people do better on the Paleo diet than others. It is clear from the feedback on this site that the diet isn't for everyone -- some people have spectacular success, and others just have problems.
I would be curious to see the results of a poll of the blood type of people that are successful with the Paleo diet. My guess is that there would be an uncommonly high number of type O's compared to the general population.