Development of Creative Commons licenses

["DeepNet" <dan AT deepnettech.com>]

Hello:

I am thinking of using the Creative Commons license
(attribution only, all other rights allowed) within a product
in a special way. I want users of the product (not under CC license)
to be able to submit documents to me, the product
owner, for possible integration into that product in
the future (and if so, I would of course observe the requirements
of the CC license, particularly attribution).

I was thinking that the user, prior to uploading, would have
to agree that they are submitting their work to us under
the Creative Commons License noted above.

Now... my question:

- I want to protect myself from people submitting works
that they DO NOT have copyright ons (i.e. they submit
someone elses work, and accept the CC license anyway).

- In effect, I would like the license to say something like,
"Licensor warrants, to the best of their ability, that they
own the copyright to the Work, or otherise have rights to 
the Work to allow this license to be valid"...

- I've read the CC license generated above, and I don't
see anywhere where the Licensor gaurantees this kind
of thing to the Licensee?

- Is this a common problem? Is it in there, or handled
in some other way? Can/Should I add a second agreement they
must also accept that simple states this as an extra thing?
Where would I get such legalese, if required?

- I understand that even if this is covered, or I add it, that I can
still be held libel for re-publishing protected works (witness
Napster, etc.). In such cases I would remove the offending
work of course. But, I want my users to explicitly see this requirement,
and be plain and up front (I imagine it as an extra bullet on
the CC license summary page, which I'm surprised it's not
there).

(if replying, please feel free to e-mail/cc me directly as well)

Thanks,

Dan Pronovost
dan AT deepnettech.com
DeepNet Technologies
www.DeepNetTech.com