How to permenently delete files...

Another method: If you are using Mac OS X and there is data you don't want others to find after you are done with it, keep those files in a RAM disk. OS X is very stable, so you are unlikely to lose such data from a crash, even if you use it for days or weeks. When you are finished with a file, erase it from the RAM disk. The RAM will be reused many times and once you turn off the Mac, those bits should be gone gone gone.

For OS 9 and earlier, RAM disks work too, but crashes are a more likely possibility, so use this trick only for short-term data (like a web browser's cache, cookies, and history).

On the other hand, we haven't discussed where Mac OS X keeps virtual memory pages. I have never studied this. When you have a file in either RAM or on the disk, might it end up in a pagefile from which it could be recovered by an expert?

A RAM disk is a disk device that exists only in the computer's memory (RAM). It acts like a regular disk device, i.e., you can see it on your desktop, open it, create folders on it, copy files to and from it, etc.

It differs from a real (physical) disk in these ways:

(1) It is orders of magnitude faster, because it runs at the speed of memory accesses rather than depending on the physical movements of a real disk drive.

(2) It uses up the memory your computer would otherwise have for the operating system and active applications.

(3) Its contents are lost when you turn off the computer, unless you first save the RAM disk contents to a file or disk image on a real (physical disk).

These characteristics make RAM disks good for certain situations. Putting a web browser's cache in a RAM disk can greatly speed up web accesses as many small files are created and deleted without using the real disk. Turn off the computer and poof - all those files are gone. Under older Mac OS releases, I would always create a RAM disk (even a small one, e.g., 5MB) for such purposes. There is debate about whether RAM disks are as useful under Mac OS X because of its advanced and automatic memory management, but I suspect they still provide benefits, including for web browser caches.

RAM disk example: Let's say you have 128MB of real RAM in your Mac. You might create a 28MB RAM disk and leave 100MB for use by Mac OS and your applications. This give you a rather tiny disk device (28MB) that is incredibly fast. But your computer otherwise operates as if it had only 100MB of RAM instead of 128MB. Having less available RAM under Mac OS 9 and earlier can limit the size or number of applications you can run. Under Mac OS X, having less available RAM may cause more disk swapping.

Prior to Mac OS X, use the Monitors control panel to create or delete a RAM disk. Under Mac OS X, you can use a utility like ramBunctious ($25) or take your chances with the various commands, free AppleScripts, or free shell scripts reported at Mac OS X Hints.

More info about RAM disks can be found in Apple's Knowledge Base and at Clarkwood Software, the maker of ramBunctious.