Use 'rm -P <filename>' on the command line. From the man page:

"Overwrite regular files before deleting them. Files are overwritten three times, first with the byte pattern 0xff, then 0x00, and then 0xff again, before they are deleted."

Also, MacOS X can do AES-256 encryption of Disk Copy images. Simply choose that option when creating a new blank image (using the Disk Copy application), and store your sensitive information in it. Don't put the password in your keychain, and make sure your password is strong. Then you needn't worry about "wiping" files you delete from the disk image, or about "wiping" the image itself when you delete it, since it's already encrypted.