Hi

Short answer:

I never heard of a checksum called "opt-out".
Also, as far as I know, there is no mechanism or technique
called "opt-out" with respect to checksums.

No comments on the 'security' of myspace, facebook et al.




Broader view, just because it's interesting:

opt-int/opt-out[nice definition: 1] has a variety of implications,
starting from software engineering up to laws, regulations and compliance.


opt-in
requires action or affirmation by an individual for inclusion. default: exclusion

opt-out
requires action or affirmation for exclusion. default: inclusion


Facebook et al:
As nihil said, there was an issue with facebook[2]
a couple of months ago - they established an opt-out issue for marketing
purposes (default: inclusion).

Mailing lists:
Usually, a double opt-in mechanism is applied (confirmation mail) with an
opt-out possibility (beware of spam-mail opt-out though ...)

Spam-Filtering:
"Grey listing" is some kind of double opt-in mechanism. If
applied correctly, spam mails can be reduced up to 90%. Well, currently ...

WebForms:
Often opt-in question are pre-set with yes (affirmation), which is
questionable practice by law in some countries (e.g. germany IIRC).
Sometime, opt-out questions are pre-set with no (ie. inclusion!), which also
is questionable practice.



Cheers

[1] http://www.inc.com/articles/2002/10/24718.html
[2] http://www.independent.co.uk/life-st...ce-761683.html