Here's another person who has reported hearing it. With a better description too (robot dying).
»forums.macrumors.com/showthread. ··· =1279821
Curiously there seem to be some who have it and others that don't. I have owned in the past a 3G and a 4 and neither of them did this.
Here's someone else questioning why his phone doesn't do it but his girlfriend's does:
»apple.stackexchange.com/question ··· ing-tone