1) Does submission of prior art guarantee it will be looked at and considered ?
2) Must a prior art submission always be associated with a particular patent or set of patents ?
3) Can one submit prior art for a patent that does not exist yet or is in pending stage.
4) Do things like video clips (say of movies), showing objects or devices that don't exist, count as prior art ?
1) No — The Examiner will probably look at it, but it is not official unless the Examiner acknowledges consideration in a communication to the applicant. Also, there are certain time limits that apply.
2) If you mean, "does the prior art need to be a patent?", the answer is no. You can submit anything that discloses the invention (for example a website printout, a book, or an article). If you mean, "must the prior art be submitted in a particular patent?," the answer is yes. You need to submit the prior art to the Examiner in each patent application.
3) Yes. To submit prior art in a granted patent, you would need to reopen examination or challenge the patents validity in legal or administrative proceedings. Prior art is supposed to be disclosed during or prior to examination of a pending application.
4) I don't know, but I think it could if that video clip disclosed the substantive technology in a meaningful way, and wasn't just a bare concept.
The rules for submitting prior art by members of the public are contained in 37 CFR 1.291.
http://www.uspto.gov/web/offices/pac/mpep/mpep-9020-appx-r.h...
It would be cool if there was a database people could contribute to, but in theory the Examiner should be able to find this stuff in their prior art searches anyway.
The reality is examiners have a very small amount of time. Somebody has to do the work of verifying submissions, and making it reliable, and they don't have time to look through large sets of maybe unreliable data.
It's not enough to just submit an idea, you need the date the idea was conceived of, as well of some verification that this occurred.