I have just put up a video on YouTube that may give you some useful tips about helping bees defend their colony against wasps (yellowjackets) http://www.youtube.com/watch?v=7JqFo5lzCPY
I have never heard of this being a problem in the US, if you maintain strong colonies. The only time I have seen yellowjackets picking on a beehive is when it was almost completely dead and eaten up by wax moth. They may have been feeding on honey for a sweet taste, but I believe that they were actually going after the brood. Being carniverous.
Squeak Creek Apiaries
Here on the west coast, southern california we do have a major problem eith yellow jackets.
We put out the pheromone traps and had to empty them every 2-3 days. The population of the yellow jackets is much lower this year.