Canon DSLR that changes aperture in Video
With canon the first DSLR that can do that is the 550d (you need to set the movie mode to manual in the settings otherwise it will adjust automatically).

However i would suggest something with a swivel screen (600d and up) because it's very useful while shooting video. The 60d and 7d have more options for video (frame rate for one) but it depends on your needs and budget if those are an option.

Hope this is helpful.