Leave the issue of formatting aside, because it isn't effective. Wiping is the method I was referring to, and it does
matter what you overwrite the old data with, and how many times. If, for example, you overwrite your entire disk once, with a pattern of all 1's, it is possible (though very expensive) to manually reconstruct the previous data. A quick Google search found this reference, as an example:
In conventional terms, when a one is written to disk the media records a one, and when a zero is written the media records a zero. However the actual effect is closer to obtaining a 0.95 when a zero is overwritten with a one, and a 1.05 when a one is overwritten with a one. Normal disk circuitry is set up so that both these values are read as ones, but using specialised circuitry it is possible to work out what previous "layers" contained. The recovery of at least one or two layers of overwritten data isn't too hard to perform by reading the signal from the analog head electronics with a high-quality digital sampling oscilloscope, downloading the sampled waveform to a PC, and analysing it in software to recover the previously recorded signal.
Granted, this is about ten years old. I believe I saw another reply in this thread stating that this was an issue only with older disks, and it isn't possible with newer drives. I don't have any specific knowledge regarding that claim, so I suppose it's possible.
In short, the point I was trying to make was that by overwriting all bits on a drive, you aren't necessarily removing all information that used to be there. Of course, overwriting more than once is overkill for most of us. If you are worried about somebody spending millions of dollars just to recover the data on one of your hard disks, you have bigger problems to concern yourself with