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If you look at what they offer, at first it appears to be a URL-shortening service similar to TinyURL only allowing you to give some context to the links so people know what they're clicking - that is until you give it a test drive.
What Tynt actually does is scrape the target page and frame it, giving it a new URL that is a subdomain of Tynt's domain. Effectively, they're scraping content from across the web that people like and encouraging users to link to the scraped content - which, for a particularly viral piece of content, could give the page more link love than the original.
This isn't just an out-an-out content scrape though because the service allows the user to add tags and notes to the page so, for example, you find a funny image on somebody else's website, you can "Tynt" it and on the "new" URL you can highlight bits of the funny image and add a comment or note. So, they could argue it's useful.
However, as Eric Lander explains quite clearly on his blog, webmasters have no way to tell Tynt to stay off their sites and Tynt's scraped content is being indexed in Google.
Even were the owners of Tynt to block spiders from this content it still presents a serious issue of copyright as webmasters will lose control of their content to a service which is not inherently all that useful but which people are likely to find entertaining - at least for a while - and is very likely to be used more for comedy and ridicule than for any type of serious discussion.
When uploading information to the internet, at what point do we have to, rather than merely making it available for commentary, agree to allow others to actively modify it for their own purposes, uploaded to another source with no clear route back to the original context?
Plagiarism and copyright violations have long been an issue on the internet and sites like Tynt look poised to make the problem even thornier by bringing scraped content into mainstream use for entertainment purposes rather than spam.