Getting the flu shot every year reduces the marginal or incremental effectiveness of getting one this year.
Probablistically getting the vaccine still reduces the likelihood of contracting the flu; it just isn't as effective as it would be if you hadn't had it for several years prior to getting it. See this. [via Jack]
Dr. Edward Belongia is among the scientists who have seen the picture coming into focus. He and some colleagues at Wisconsin’s Marshfield Clinic Research Foundation reported recently that children who had been vaccinated annually over a number of years were more likely to contract the flu than kids who were only vaccinated in the season in which they were studied.
The article goes on to emphasize that getting the shot is still effective (versus not getting it), but just not as effective.