does that mean that the puppies not will have SM?
The short answer as Cathryn notes, is NO. This condition is so widespread in the breed now that genome researchers could not even find enough totally clear cavaliers (clear of the malformation that causes SM, and SM itself) to form a crucial clear genetic group that would help them find the responsible genes in affected dogs. Almost every cavalier has the malformation, which causes the skull to be too small for the brain, in different degrees of severity. They have had to go to a totally different breed that also has SM in smaller numbers, Brussels Griffons, to find the clear control group for the very important gnome research now taking place in Canada. You can read more about where this research stands
here.
What that means is that as with MVD the genes for the condition are likely permeating nearly all cavaliers so almost any mating is going to pass along some of those genes. What isn't understood yet is the mode of inheritance -- if it is a polygenetic trait (eg requires several genes to combine to produce affected dogs) as researchers believe, then which genes or possible environmental influences trigger more severe cases?
What initial research has shown is that A grade dogs -- no syrinx -- tend to produce almost no affected and symptomatic dogs, and the highest number of A graded puppies (though these have not been tracked across a full lifespan as this work is only a couple of years old now). Also, scans of family groups have shown that A grade dogs tend to be closely related to other A grade dogs -- find one A grade dog and you'll probably find others amongs parents or siblings or offspring. So scanning has enabled breeders to find other A dogs very successfully in the Netherlands (instead of just randomly scanning in the hopes of finding As), where they are very organised in doing such research (sadly no other breed clubs are pursuing this type of research at this time though the UK club is sponsoring research to get clearer breeding guidelines). There are known, affected offspring however from AxA matings but not many. Statistically this is always possible.
Asymptomatic but affected dogs (that turned out to be D grade or eventually symptomatic) have so far produced NO A grade offspring and a significant number of low grade dogs.
You can listen to a free podcast of Dr Clare Rusbridge explaining the condition and this research here:
http://sm.cavaliertalk.com/rusbridge.mp3
You can buy two CDs of all the talks (aimed at a general audience) at the recent conference on SM in cavaliers in Rugby, UK here:
http://www.cafepress.com/cavaliertalk/4311456
Personally, I would now want scan grades for both parents and to discuss overall breeding programmes before buying any puppy.