What they don't tell you is that doing so is extremely time consuming and expensive, and results in a jumble of isolated files or even just blocks that then must be pieced back together. Very expensive to do and very unlikely unless there is an extremely good reason someone wants to recover that data.
There are disk washing programs that will overwrite every byte of data on the disk effectively rendering it wiped without the use of expensive laboratory-type procedures.
Yup. While information may be recovered, it's a cost-vs-profit ratio issue. Just deleting everything then overwriting everything is enough unless you have some VERY valuable data to eliminate.
Under most conditions, it's far easier to resort to "rubber hose crypography" than CSI/NSA-type data recovery.
Haven't had to do it recently, but if I remember correctly a good low level format (done by BIOS) changes every bit to zero effectively cleaning the drive. Some information may still be stored in hidden sectors, but it won't be your every day skeletons. The last time I did that it took all night on an old 512mb HD.</p>