I'd be very cautious about using flexible magnets to mount anything, especially anything holding anything sharp. They tend to not be very strong (as compared to some other magnetic materials). Even with the strongest magnets though, as Jeekinz said, sliding WILL occur (unless the magnets are so strong that they won't slide... but then you won't be able to get your knives off either).
You could make your own rack... I would suggest using Neodymium magnets instead of anything you might find in a hardware store or salvage from something else to do it though... They're strong stuff, and one of these little buggers goes a long way.
United Nuclear - Neodymium magnets
What you could do is get some the 1/4" x 1/4" long Rod magnets. Get three sheets of 1/4" thick plywood about 10-12" long and 1" high. Drill some 1/4" holes in one of them. (two holes vertically, 12 holes horizontally, along the center of the board. which equates to a hole every inch or so along the length). Fasten this holed board to one of the other sheets. Fill the holes with magnets. Fasten the other board to the other two so the magnets don't fall out. Affix the board strip under a cabinet or to a wall (you might need to use additional spacing for wall clearance, just add wood to the back).
You could add whatever you want to prevent your knives from hitting the wood if you'd prefer... Some veneer or something.