Hi Chris,
The URL makes it neither secure, nor unsecure. the real security is on the form itself, by validating that the current user has rights to the record before writing it.
That said, the newer approach, of using a hashed unique ID is "more secure" in that the actual ID field is unknown. This needs to be further closed down though so that the actual ID, if used (like mentioned on FAQ W4) will fail.
Coupled with that is the need to document the technique for calling a form directly, but using the new hased-id approach rather than the old id approach. That's not documented yet.
cheers
Bruce