Since I tend to connect and disconnect my Android phone to my Mac pretty often, I got frustrated that it kept launching Aperture every time (it used to do the same with iPhoto, before I switched).
In any case, the solution for this is easy enough; with the phone connected, launch the ImageCapture.app: select the phone, and choose "No application" from the drop down list in the bottom left corner:
This is under Lion; your mileage may vary under earlier versions. Hope it helps!
