Why buy SD (specifically MiniDV)? Because, speaking in generalities:
1. DV cameras are cheaper, and HD cameras will only come down in price the longer you wait. $1400 is a nice chunk of change to spend on a consumer camera. $700 today on an DV camera and $700 3-4yrs from now on an HD camera might be money better spent.
2. DV cameras have better low light performance.
3. DV doesn't use an interframe compression scheme so the image is less likely to breakdown due to fast movement in frame or fast movement of the camera.
4. HDV for use in iMovie (and FCE, IIRC) requires a transcoding step that makes capturing longer than realtime.
5. HDV requires a reconform render at the end before you can put your footage back to tape.
6. DV cameras (all most all of them) will allow you to monitor your video on a TV while editing (computer->Firewire->camera->analog->TV) which can be very helpful since TVs and computers monitors display the images differently. Because of reason number 5 you cannot use an HDV camera in this fashion.
All though I don't know for sure, I think AVCHD cameras suffer similar shorting comings as HDV cameras (reasons 3, 5, 6) because they both use interframe compression schemes. Of course, AVCHD, AFAIK is not yet support by very many NLE's including all of Apple's.
Lethal