While Dust is relatively A) a non factor in the overall quality of the image unless it's grossly saturated and B) a factor of life, there's a few things to think about (regarding lenses). On the weathersealed L lenses, theres a disclaimer that they are weathersealed IF you put a filter on the front. I dont know much about how that works with non weathersealed lenses, however if that's a requirement for the best lenses and filters, then I'd say it's probably a good idea to consider that if you have a high quality filter you can slap on the front.
Also, as neuro stated, it's a relative non factor in the overall image and should some get in whether it from the front element, rear element, barrel, or any other nook or cranny, it isn't something that you should get worried about and if you happen to qualify for CPS, you can get your gear cleaned for free a few times a year anyways. I did read a while ago that this lens with the frequency and location on the vent holes has a higher dust problem than most lenses, however unless you'r taking the lens to the sahara's or shooting in the middle of a dust storm, I dont see many situations in which this lens would be rendered useless.