Wow, looks like a string of bad luck for you, mate.
5 DSLs? Switch to a different color or something man
1 DSPhat (red) never had any problems -> sold to friend
1 DSLite (onyx/black/whatever) never had any problems -> still own
In fact, I only know of one person that had a crack, and it was a white/pearl one they got launch day here in the US, and the crack was barely noticeable, i told him about sending it in to get fixed, he said "that'd be fine, but 1. it doesn't effect usage at all, it's just a cosmetic thing 2. he would be without a ds for awhile, and because of those two things, he played on.