Yes, I just noticed the same problem with .AVI's imported from my Canon S1 IS. I was bewildered until finding your question, which pointed me to the answer -- just look in your iPhoto folder under a date for which you have some AVI's and you'll see that what's there are actually a .jpg's (not .JPG, as the photos imported from my camera are named) rather than the AVI's you expect. Apparently iPhoto generates a jpg for the (first frame of the?) movie and moves the movie itself to the "Originals" folder.

So, how do you make a smart album for your movies? Fortunately, the movies imported from my camera all have file names beginning with MVI_, so instead of specifying "ending with .AVI" I specified "beginning with MVI_". (I had already made a smart album for components of a panorama, which happen to get imported as files whose names begin with "ST".)