I thought that too but late last year one of these Bradley Manning articles seemed to suggest that data might be able to persist after a simple zero-fill, so I thought I'd use my prng script before zero-filling, because it's as fast as zero-filling and it seems unlikely you could decipher more than two layers.
I looked up the article; the statement is actually more ambiguous than I recall:
"Johnson testified that he found two attempts to delete data on Manning’s laptop. Sometime in January 2010, the computer’s OS was re-installed, deleting information prior to that time. Then, on or around Jan. 31, someone attempted to erase the drive by doing what’s called a “zerofill” — a process of overwriting data with zeroes. Whoever initiated the process chose an option for overwriting the data 35 times — a high-security option that results in thorough deletion — but that operation was canceled. Later, the operation was initiated again, but the person chose the option to overwrite the information only once — a much less secure and less thorough option. All the data that Johnson was able to retrieve from un-allocated space came after that overwrite, he said."
It's not clear what "after" means exactly, but it surprised me that he learned about the first attempt after the second attempt was complete. What may have happened -- making this much less impressive -- is that maybe /dev/sda1 was filled with /dev/zero rather than /dev/sda, so that the first part of the drive was totally untouched. So it might be much less impressive than it sounds, but it sounded impressive to me at the time.
Or you could have used DBAN, which has modest min spec requirements.