Using rm to delete files is just begging people to skank around your disk to read the old data, which is no good if you really want your files to disappear forever. Fortunately for all of us, there's the shred command, which overwrites your file with new data multiple times before deleting it, which makes recovery absolutely impossible.
For example, if you wanted to be absolutely sure that the file passwords was removed, you would use this command:
shred -z -u passwords
That overwrites the file with random data 25 times, which is then followed by a final zero pass (-z) so that no one can tell you've shredded something, then removes it (-u).
The removal isn't enabled by default, because you can tell shred to work on entire partitions (eg shred /dev/hda1), in which case you probably don't want it removed
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