Many businesses perform 3 pass deletes/wipes. I have used programs like Access Data's Forensic Tool Kit and have been unable to recover data after 1 pass. Like what Tom said, when the OS deletes a file, it just marks the file header as being blank and does not actually delete the data, but doing a PGP Shred or a full wipe does delete the data.
I have also heard the same, that some government agencies can recover data by doing a surface scan. I have had conversations with representatives from Seagate and also from Drive Savers with the same answers on how to destroy disks and data on disks...smashing with a hammer, drilling a hole through the platters, etc... On the other hand, I have spoken to people in more of the forensic/law industry that say they have recovered data from disks with holes drilled in them as well as disks that have been through fires.
In my opinion, unless you are dealing with a government agency that really wants to get to your data, a single pass wipe/delete would be good enough. If you are dealing with one of these agencies, making sure it is whole disk encrypted while using it, and then physically shredding the disk (or some other method of physically destroying the platters) would be the way to dispose old disks.
Also take note, it seems to me that it would be rather difficult to do a surface scan on a drive to recover a file that has been WDE'd as well as PGP shredded because the data recovered from a surface scan would need to be decrypted as well.