Since this was posted in '06, has no one else had this question? I can scp files to the ASA and can even scp files from disk0 on the ASA to/from a linux box. I cannot however figure out how to grab system:running-config to get the config the same way you would from a tftp. This is for backup purposes to replace tftp.
As I mentioned in my earlier post, you could setup a VPN between the ASA|PIX and the Server running the TFTP daemon to secure the communication, I would give it a try. You can use transport mode for this purpose for minimal overhead.