It's at least a one-pass overwrite with random data. Any more than that would be an absurd waste of time.
Can anyone tell me what level of security is secure delete? (I mean securely emptying the trash.)
It's a 7 pass erase approved by the Department of Defense and the NSA for hard drives.
Since it's usually done for small files, it doesn't take a long time, however if you use Disk Utility > Erase Free Space and use the 7x pass, it can take quite a long time, days even.
The Erase Free Space > Zero is a one pass and designed to stop software like Data Rescue from reading the deleted bits that are due for a overwrite if more data comes along to be written.
If your selling the machine, you can Zero bit erase the entire partition or drive and reinstall OS X brand new with the video and everything.
Page 162, NSA - Mac OS X Security Configuration for Version 10.6 Snow Leopard
It's always been a 7 pass to meet government requirements.