Before I begin, I think I should mention a few basic things about intellectual property law since it seems to be somewhat misused in this thread. BTW, to the chagrin of some who seem to think that suing is so easy, think again. Lawsuits take at least a year before they get to court, and with lawyers charging $400/hr, think how much that can add up to.
Copyright - A copyright protects works of expression, such as art, music, dramatic, and architectural work. For example, every flash submitted to the portal is eligible for copyright protection. And every song you download has a copyright protection.
Patent - A patent protects any new and useful process, invention, or machinery. The George Forman Grill, disposable boxer shorts, and the Amazon.com "one-click" ordering system would be eligible for patent protection. So it is likely that the flash portal would also qualify for patent protection.
Do you see the difference? Mainly these laws are here to make sure that the original composer or inventor gets rewarded for their creativity.
However, since as a social benefit, it is better in the long run to have free information, there is a limit to the protection to patents and copyrights. Patents are protected for 20 years and copyrights for 70 years after the author's death.
Tom is correct from a social perspective. If something is good, then people tend to create their own versions of it. Look at reality TV. It began with Survivor and now it has turned into wife-swap and a show where the contestants fight each other to adopt a baby.
But having other portal sites also promotes competition which in the long run is good for others. Other people may have cool ideas too. There is a subtle difference between blatant copying and putting a twist on an existing idea. Based on Tom's rant, it seems that there is some sort of tacit approval among the webmasters to let each other be. If that's the case, then by all means go with it. Im sure that NG can learn from other portal sites just as other sites have learned from NG.
The point is, copying is not really a big deal. Copycats are just trying to accomplish what the creators of NG has done: become popular internet personalities. If all of the previous posters are correct, then this site, with its huge following should remain extremely popular for a long time. But more users also means more POS works.
The only time copying is a a big deal is when money is involved, as Im sure the RIAA has shown. To the NG staff, I dont know what Newgrounds means to you financially, but if you think this may be an issue, I suggest that you speak with an intellectual property attorney. I highly recommend going to www.ipwatchdog.com. There's a lot of useful information there and the webmaster was my patent law professor.